The simultaneous non-contact measurement of the front and rear surfaces of parallel plates by using wavelength phase-shifting interference technology was of great significance in the field of optical determination. A time-domain weighting algorithm was proposed to solve the overlapping interference signals of the front and rear surfaces of parallel plates: weighted 36-step sampling algorithm. Firstly, based on the principle and constraints of the algorithm, the basic distribution parameters of the weighted multi-step sampling algorithm were designed, and then the sampling weights of the interference signals of front surface, rear surface and thickness variance were obtained. Through the calculation of the basic distribution parameters, the sampling weight of each surface can be obtained, and the initial phase distribution of each interference information can be obtained by using the weight operation. Based on Zernike polynomials, the simulation of multi-surface interference phase solution was carried out, and the maximum error between the simulation result and the true value was not more than 0.06 nm. In addition, the influence of various errors on the measurement results was analyzed. A parallel plate with a thickness of 20 mm has been measured. The experiment verifies the effectiveness of the algorithm in the actual measurement process.