Tire pressure monitoring system (TPMS) is an electronic system monitoring the air pressure in a pneumatic tire at real time. The goal of this paper is to analyze and optimize the design of a novel wireless passive SAW sensor used in TPMS, via theoretical derivation and simulations. For this purpose, a P- matrix with new expression is derived utilizing the coupling-of-modes (COM) theory. The applicability and accuracy of the new P-matrix expression in analyzing the unitary symmetry IDT and the reflector have been validated. The analysis of the insert loss (IL) of the novel SAW sensor based on this P-matrix model reveals that this design has excellent performance after the parametric optimization.