This work has involved the simulation of technological operations of manufacturing a silicon (Si) avalanche photodiode and calculating its output characteristics. As a result, the values of its multiplication factor (from 49.5 to 63.3) and spectral sensitivity (from 15.8 to 19.6 A/W) depending on the shallow trench insulation defect type are presented for a Si avalanche photodiode. It has been shown that the technological defects causing narrowing of the width of the trench towards its bottom have the greatest impact on these parameters. As a result, the multiplication factor decreases to 18%. The dependence of the photodiode gain on the trench’s wall inclination angle was established to be described well by a linear function. It was also demonstrated that the trench narrowing led to an increase in the lateral component of the electric field strength near the groove, which reduced the breakdown voltage, but did not lead to an increase in multiplication.