For a coherent optical fiber transmission system that fully compensates for the signal-signal nonlinear interaction, the communication performance of the system is mainly limited by the nonlinear signal-noise interaction (NSNI). The theoretical mod-el of the full-order NSNI is proposed, and the full-order NSNI model is verified by taking a Nyquist optical transmission system with a rate of 256 Gb/s and a 16-order quadrature amplitude modulation(16QAM) signal based on polarization multiplexing as an example. The experimental results show that only considering the first-order and second-order NSNI effects will overestimate the communication performance of the system, while the full-order NSNI model can more accurately reflect the influence of the NSNI effect on the system.