According to the requirements of high-energy particle detection for a large photocathode detection surface, low transit time spread (TTS), and low-cost photomultiplier tube (PMT), a kind of 20-in. hybrid large-area PMT based on the silicon (Si) electron multiplier array was designed and optimized. This study tracked the trajectories of photoelectrons from the photocathode to the silicon electron multiplier array based on the Monte Carlo and finite-integral method. The critical effects on the TTS characteristic of the large-area PMT, including the focusing electrode structure, glass shell structure, different potential differences, and relative distance from the photocathode vertex to the silicon electron multiplier array, were studied in detail. After optimizing the structure of the glass shell, the 20-in. hybrid PMT based on the ultra-small Si electron multiplier array with 40 mm collection diameter can achieve an excellent TTS of about 1.87 ns from the photocathode to the Si electron multiplier array at a collection potential difference of 2000 V. © 2022 Author(s).