The invention provides a multi-wavelength optical measurement device and measurement method thereof. The multi-wavelength optical measurement device includes a laser generation module, an optical module, a reflection light receiving module and a control module. The multi-wavelength optical measurement device can generate two or more lasers having different wavelengths. The reflectivity of the lasers of different wavelengths is not smaller than a set threshold value at the same time. Through switching, lasers having different wavelengths are taken as an incoming light source by the multi-wavelength optical measurement device and a condition is ensured that the current reflectivity of laser of the current wavelength is always greater than the set threshold value. According to the invention,alternative switching of the lasers having different wavelengths is utilized for optical measurement, a condition that the reflectivity of laser subjected to current detection is comparatively high isensured all the time. Since that the reflectivity of the lasers having the different wavelength may not approximate to zero at the same time is ensured, warping rate measurement validness is ensuredat any phase of thin film growth.