Regarding the design problem of heliostat field in the tower solar thermal power system, the corresponding efficiency calculation and parameter optimization models are established based on planar optical laws and layout strategies. Firstly, the Monte Carlo method is employed to establish computational models for shadow occlusion efficiency and truncation efficiency, thus obtaining the computational model for the optical efficiency of heliostat field. After comparing different optimization strategies for heliostat field layout, the occlusion-free layout is chosen as the optimization strategy. An optimization model for heliostat field parameters meeting the output power requirements is then proposed. The variable step size search algorithm is employed to estimate the heliostat field parameter values. Finally, based on the model calculation, the optimized heliostat field achieves an average annual optical efficiency of 63%, an average annual shadow occlusion efficiency of 94.9%, an average annual cutoff efficiency of 78.17%, and an average annual thermal output power per unit heliostat area of 0.7312 kW/m². Compared with the performance of the heliostat field before parameter optimization, the performance is significantly improved. For instance, the average annual thermal output power after optimization is 60.2933 MW, which represents an increase of 91.57% compared to the pre-optimization level.