In this paper, we propose a simple yet effective method to measure the operating wavelength of a laser using laser self-mixing interferometry (SMI). Analyzing the self-mixing interference signals acquired from external objects with known motion patterns allows us to deduce the laser's operating wavelength. To validate the accuracy of our method further, we constructed a laser self-mixing linear motion sensing system. This system captured the self-mixing interference signals generated as external targets moved at a uniform linear velocity under various laser wavelengths. By conducting FFT analysis, we obtained the signal frequency, which was then converted from laser Doppler frequency to optical frequency, ultimately determining the laser's wavelength. Our experiment specifically focused on analyzing the impact of frequency doubling on the results, discussing its causes and potential solutions. Numerical analysis indicates that the laser's working wavelength parameter is precisely measured from external targets moving at a uniform speed of 0.4 to 2.4 mm/s. Additionally, the experimental results show a absolute error of less than 1.230 nm and an relative error of less than 0.12 %. In conclusion, our experimental, simulation, and theoretical analysis results mutually validate each other, confirming the accuracy and practicality of the measurement method.