Under regular waves, as the wave parameters change, the likelihood of capsizing decreases, and the number of normal sailing times of the ship significantly increases. In order to ensure that the test results meet the requirements of statistical reference, the capsizing model test under irregular waves will be repeated many times to increase the reliability of the test data. This article uses the method of describing regular waves, improves the control equation, combines P-M wave spectrum and Newman wave energy spectrum, and proposes an approximate way to use regular wave equation to describe the motion law of ships under irregular waves. The concept of stability criterion is introduced to express the probability of ships riding waves and swaying under irregular waves. Taking the ITTC ship A2 fishing vessel as an example, this study investigates its motion state under different initial speeds. The research results can provide technical support for the complete stability of the second generation.