The current non-destructive testing of ammonia fuel engine surfaces using laser ultrasonic Lamb waves result in severe frequency dispersion and multimodal phenomena, making the signal complex and unable to completely eliminate the dispersion effect, which has always been a challenge in this field. For this reason, an improved method for detecting corrosion damage in ammonia fuel engines based on laser ultrasonic Lamb waves is proposed. Firstly, a linear mapping dispersion compensation method is introduced to effectively correct the dispersion effect in the one-dimensional laser ultrasonic Lamb wave signal captured by laser excitation and reception on an ammonia fuel engine. Secondly, the amplitude spatial wavenumber spectrum of the frequency, wavenumber, and spatial position information of the signal at a specific center frequency is obtained through short space two-dimensional Fourier transform to determine the approximate location of corrosion damage on the scanning path of the ammonia fuel engine. Finally, based on signal characteristics, the RAPID method is used to determine the precise location of engine corrosion damage and achieve corrosion damage detection. The experimental results show that after compensation of the proposed method, the signal waveform is significantly improved, with compact time-domain and clear frequency-domain characteristics, and the proposed method can effectively improve the accuracy and reliability of corrosion damage detection in ammonia fuel engines.