NOVELTY - Combined fertilizer synergistic agent comprises phosphorus solubilizing microbial inoculant, potassium solubilizing microbial inoculant and polyglutamic acid according to ratio of 0.1-1:0.1-1:0.001-0.1. The phosphorus solubilizing microbial inoculant comprises Bacillus subtilis having strain number of JT-1 and preservation number of CCTCC M 2014367, which is preserved in China Center for Type Culture Collection on August 5th, 2014. The potassium solubilizing microbial inoculant comprises B. subtilis having strain number of JT-2 and preservation number of CCTCC M 2014368.    USE - The fertilizer synergistic agent is useful: for growing crops including grain crops, fruit and vegetable crops, or melon and fruit crops through hydroponics, foliar spraying, irrigating or hole fertilization process; and as fertilizer (all claimed).    ADVANTAGE - The fertilizer synergistic agent: utilizes phosphorus solubilizing microbial inoculum, which releases phosphate ion in the soil; potassium solubilizing microbial inoculum, which releases potassium ion in the soil; provides favorable survival growing environment; improves the utilization of fertilizer; and can be directly used as fertilizer.    DETAILED DESCRIPTION - Combined fertilizer synergistic agent comprises phosphorus solubilizing microbial inoculant, potassium solubilizing microbial inoculant and polyglutamic acid according to ratio of 0.1-1:0.1-1:0.001-0.1. The phosphorus solubilizing microbial inoculant comprises Bacillus subtilis having strain number of JT-1 and preservation number of CCTCC M 2014367, which is preserved in China Center for Type Culture Collection on August 5th, 2014. The potassium solubilizing microbial inoculant comprises B. subtilis having strain number of JT-2 and preservation number of CCTCC M 2014368, which is preserved in China Center for Type Culture Collection on August 5th, 2014. An INDEPENDENT CLAIM is also included for preparing the combined fertilizer synergistic agent, comprising (i) preparing phosphorus solubilizing microbial inoculant by activating inclined plane phosphorus solubilizing bacteria, inoculating the activated bacteria in a culture medium, culturing for 18-24 hours, selecting the activated culture strain, then inoculating into a fermentation culture medium, culturing at 30-35 degrees C for 12-24 hours to obtain fermentation liquid or drying fermentation liquid to obtain powder; (ii) preparing potassium solubilizing microbial inoculant by activating inclined plane phosphorus solubilizing bacteria, inoculating the activated bacteria in a culture medium, culturing for 18-24 hours, selecting the activated culture strain, then inoculating into a fermentation culture medium, culturing at 30-35 degrees C for 12-24 hours to obtain fermentation liquid or drying fermentation liquid to obtain powder; and (iii) uniformly mixing the microbial inoculant obtained in the step (i), microbial inoculant obtained in the step (ii) and polyglutamic acid according to weight ratio of 0.1-1:0.1-1:0.001-0.1.