In order to solve the problem of poor precision in observed target, measured by manual focusing in traditional shooting range optical measurement imaging device with low efficiency and indeterminacy, an algorithm of rapid auto-focusing combining roughly focusing and fine focusing is proposed. This method can automatically search the focal plane by evaluating the image definition. First, the Brenner function with high performance is used to roughly fousing. Tenengrad function with high sensitivity to the change of image definition is used to fine fousing in narrow range later. An algorithm for tracking based on mean shift is applied and hardware platform is set up in experiment. By the focusing experiments repeatedly to the same measuring target in the same place, the results of field test indicate that this method with high stability can satisfy the accuracy and velocity in actual measurement.