NOVELTY - Preparing hydrogen in situ by using natural gas hydrates, comprises: (a) performing decomposition of natural gas hydrate to obtain water steam and natural gas output, preheating the gas after mixing, and contacting with a steam reforming catalyst; (b) after cooling the obtained product gas, contacting with a carbon monoxide conversion catalyst; and (c) subjecting the obtained transformed gas to membrane separation and purification, to obtain a hydrogen gas containing less than or equal to 50 ppm carbon monoxide and separating an exhaust gas, and after cooling, entering hydrogen gas into hydrogen fuel cells.    USE - The method is useful for preparing hydrogen in situ which is useful for a hydrogen fuel cell vehicle (all claimed).    ADVANTAGE - The method is economical and does not require to build large number of hydrogen refueling stations.    DETAILED DESCRIPTION - Preparing hydrogen in situ by using natural gas hydrates, comprises: (a) performing decomposition of natural gas hydrate to obtain water steam and natural gas output, preheating the gas after mixing, and contacting with a steam reforming catalyst to prepare product gas, in which the reaction pressure is 0.1-2 MPa, reaction temperature is 450-1200 degrees C, methane space velocity is 500-5000 hour-1, and water to carbon ratio is 2-6; (b) after cooling the product gas, contacting with a carbon monoxide conversion catalyst to prepare transformed gas, in which the reaction pressure is 0.1-2 Mpa, reaction temperature is 300-500 degrees C, and the space velocity is 500-2000 hour-1; and (c) subjecting the transformed gas to membrane separation and purification, to obtain a hydrogen gas (containing less than or equal to 50 ppm carbon monoxide) and separating an exhaust gas, and after cooling, entering hydrogen gas into hydrogen fuel cells. An INDEPENDENT CLAIM is also included for a system for in situ production of hydrogen using natural gas hydrates, comprising a natural gas hydrate supply device, a steam reforming device, a carbon monoxide conversion device, a device for membrane separation and purification of hydrogen, a cooling device and a hydrogen fuel cell, where: the gas hydrate supply device is connected to the steam reforming device which is connected to the carbon monoxide conversion device which is connected to the device for membrane separation and purification of hydrogen which is connected to the cooling device which is connected to the hydrogen fuel cell; the natural gas hydrate supply device comprises a natural gas hydrate reservoir, a heating assembly, a natural gas pipeline, a decomposing water reservoir and a steam pipeline; and the steam reforming device includes a preheating assembly, a cooling assembly, a combustion heater and a reactor; the reactor is tube type fixed bed reactor, depositing with an active component of the steam reforming catalyst; the inlet of preheating assembly is connected with natural gas pipeline of natural gas hydrate supply device and steam pipeline, respectively; the outlet of preheating components and inlet of reactor are connected; the inlet of the cooling assembly and outlet of the reactor are connected; the outlet of cooling assembly and inlet of carbon monoxide conversion device are connected; and the fuel pipeline of combustion heater, natural gas supply device and/or cooling device are connected.