A composite biocontrol microbial agent PSX combination controls several soil-borne diseases. The PSX combination was composed of three biocontrol bacteria, which are shaking cultured in the PDA culture solution at 37° C., 180 rpm for 20 to 24 hours, then centrifugated at 4000 g for 10 min, taking the wet bacterium preserved fluid by the ratio of 1:40 to prepare the microbial agent, and then mixing at the ratio of 1:1:1, the total concentration of live bacteria in finished product being of 1×109˜1×1012 cfu/mL. The PSX combination controls soil-borne diseases of plant that occurs complexly or solely, including root knot nematode disease, bacterial wilt, verticillium wilt and blights. Its disease-preventing effect is 60-100%. Yield increase is 30-200%. The effects of preventing Fusarium wilt of banana and rice sheath blight disease are more than 50%, and have 20-80% yield increase. In addition, the PSX combination also can improve soil hardening.