In a stimulated emission depletion (STED) microscopy, a periodically arranged optical lattices is used as fluorescence depletion pattern to achieve parallelized multi-point fluorescence depletion, which can effectively improve the time resolution of imaging. We propose a simple optical system model of microscopic imaging system in parallelized STED microscopy and derive out a formula to illuminate the effect mechanism of tube lens and microscope objective on parallelized fluorescence depletion pattern periodicity in the model. Then we work out the optimum optical parameters which are used to generate a parallelized fluorescence depletion pattern with smaller periodicity from our formula. The numerical simulation result shows that square lattice-like parallelized fluorescence depletion pattern with a periodicity as small as 276 nm× 276 nm was generated with our method.