For suppressing the error from the incident angle fluctuation during laser thickness measurement online, the principles of laser transmission and reflection thickness measurement were analyzed by geometrical optics, which show that when the fluctuation of laser incident angle is nearby a particular value, the thickness measurement errors of two methods are complementary with one positive and the other negative. On this basis, a complementary thickness measurement method was proposed based on laser transmission and reflection simultaneous measurement, and the error could be limited between the transmission type and reflection type measurement errors, suppressing the online thickness measurement error. For the polymethyl methacrylate sheet, theoretical calculation results show that when the fluctuation span of the incident angle is at 67.013±4 degrees, the relative error absolute value is within 1% and the mean error suppression ratio is greater than 90%. When the incident angle is 61.536 degrees, the error suppression ratio is 100%. Using one line structured light laser and two linear array CCD cameras, a complementary thickness measurement system was set up. The polymethyl methacrylate sheets with the nominal thickness of 1~5 mm were measured. In addition to the 1mm glass, the complementary thickness measurement error is restrained between the transmission and reflection measurement errors, and the maximum error suppression ratio is 61%. Experimental results show that the complementary method can effectively restrain the error and improve the online thickness measurement accuracy.