Starting from the four-level system rate-equations, the relationship between output parameters and pump parameters in multi-mode solid-state laser has been deduced. For a three liner laser diode side-pumped symmetry in laser diode-pumped solid-state lasers, the pump intensity distribution is calculated, and series numerical calculations relating to the relationship between output parameters (such as threshold pump power, slope efficiency, output power and beam performance) and pump parameters are conducted. It can be seen that both radius of Nd∶YAG crystal and of pump beam waist are main factors to affect output power and beam performance. The beam performance has been improved obviously by mode-selecting with a small aperture. A set of parameters in three Liner laser diode side-pumped symmetry have been chosen to test the numerical calculation results experimentally. The experimental results are consistent to calculated ones.