On the premise of guaranteeing the image quality of imaging sona r and alleviating the contradiction between focused precision and computational complexity, a timevarying focused multi-beam forming method based on distortion evaluating is proposed . Firstly, the focal points of time structure are combined with the spatial structure of semicircular array to obtain accurate phase information of all focused points through near-field time-varying focus model. Then the optimal weights are designed by using second-order cone programmin g(SOCP), which balances various performance indicators of beam pattern. Finally, some necessary focused planes are selected by a kind of evaluation model, which is established to evaluate the degree of beam distortion. The simulation and experimental results show that computation and storage requirements are alleviated, engineering application can be realized easily, and high image quality can be achieved for complex and changeable scenes by the method.