The invention discloses a method for measuring the backlash of small modulus gears. The driven gear is rotated to the state where the test surface is horizontal, and the initial angle θ is determined. 0 is 90 degrees, and the probe of the dial gauge is vertically against the test initial point on the surface of the above-mentioned test surface, and the position data of the test initial point is recorded as (x 0 ,y 0 ), so that the value of the dial gauge returns to zero, at this time y 0 =R, turn the driven gear until one surface of the driven gear meshes with the driving gear, record the current data △y of the dial indicator, and substitute the known parameters R and θ into the formula 0 , Δy, x 0 , get the normal inclination angle θ of the test surface 1 ; Then by knowing the radius of the gear pitch circle as r, measure the gear backlash l: l=r·|θ 1 ‑θ 0 |, the present invention does not need to extend the probe of the dial indicator into the gear gap, and can accurately measure the gear backlash of small modulus, the realization method is simple and convenient, does not require too many auxiliary test tools, and there are few parameters to be tested. The cost is lower.