Based on the background of multitarget detection in the lidar field, a multibeam formation method for a silicon-based optical waveguide phased array is proposed and the working mechanism of the optical waveguide phased array is introduced. Based on the principle of beam deflection, the total aperture array is divided into several continuous subapertures by the subaperture method. After passing through each subaperture, the beam will be subjected to different phase modulation. Finally, various beams that can be deflected through different angles are formed at the exit end. According to the target threat degree, the number of array elements of the subaperture is adjusted and the multiple beams are divided uniformly or non-uniformly. Then, the energy of each beam can be reasonably distributed. Simulation results show that the far-field diffraction pattern appears at the desired angle, verifying the feasibility and effectiveness of the proposed method. Besides, the proposed method can significantly increase the number of targets detected by a phased array lidar and reduce the scanning time.