In order to realize the real-time measurement of haze and light transmittance for large-surface transparent samples, a haze and transmittance tester using LED as the light source and a double integrating sphere as the signal measurement part was designed. First, the theoretical derivation of the light transmittance and haze was completed on the basis of the system scheme. Second, simulation analysis of the light source spectral distribution affecting measurement, optical design, integrating sphere parameter design, and support adjustment mechanism design were completed, increasing the test accuracy and efficiency. Finally, calibration and sample testing of the haze and transmittance real-time tester were conducted. The test results show that the transmittance absolute error of the standard haze sample is less than 0.4% and its haze absolute error is less than 0.3%. Moreover, the transmittance deviation between the measured value and theoretically calculated value of the samples from the subscriber is also less than 0.4%, which confirms the claim of the subscriber.