Abstract: Mycorrhizal helper bacteria（MHB）has effect on promoting the plant growth. In order to discuss its biocontrol function to soilborne fungus pathogen, we used the plate confrontation in vitro and in vivo inoculation methods to assay the inhibition effect to soilborne fungus of 4 MHB isolates. The result showed that, in plate confrontation in vitro experiment, Bacillus cereus HB12,HB59 isolates and B. pumilus HR10 isolate had the obvious inhibition effect on Rhizoctonia sp, which caused the seedling dampingoff disease. In these isolates, B. pumilus HR10 isolate had the strongest inhibition effect, with 85.58 % inhibition rate. B. cereus HB12, HB59 isolates, B. pumilus HR10 isolate, and Micrococcus luteus HR15 isolate all had no inhibition effect on seedling dampingoff pathogen Fusarium oxysporum. Then the screened B. pumilus HR10 isolate was chosen for in vivo inoculation test, the result indicated HR10 had the better biocontrol function to P. thunbergii dampingoff disease with only 15 % disease incidence and 10 % mortality after inoculation, while the control had 80 % disease incidence and 65 % mortality, with just inoculate Rhizoctonia sp.. Based on the above results, MHB not only had the growthpromoting effect on plant, but also had biocontrol function to some kind of plant soilborne fungus pathogen. So it could serve as a valuable reference for further investigation on tree nursery management practice.