The modulation transfer function (MTF) value of complementary metal-oxide-semiconductor (CMOS) image sensors pixels at Nyquist frequency is typically used to evaluate the imaging quality. The MTF is mainly calculated by taking the frequency domain product of the aperture MTF and the diffusion MTF. The aperture MTF is determined by the physical structure of the CMOS image sensor pixel, whereas the diffusion MTF is determined primarily by the process parameters of the p-n junction in the photosensitive region. Simultaneously, the process parameters will affect the quantum efficiency (QE) of pixels, which will then affect the signal-to-noise ratio (SNR). The theoretical mechanism of MTF function and SNR is examined in detail in this paper, and the calculation results of MTF and SNR of CMOS image sensor pixels under 8 typical optical wavelengths in the 300--1000 nm spectral band are listed. The MTF of the pixel with L-shaped sensitivity aperture at Nyquist frequency is fixed at 0.67, the diffusion MTF at Nyquist frequency decreases as incident light wavelength increases, the peak value of QE is 85.8% at 800-nm incident light wavelength, and the peak value of SNR is 124 at 800 nm under the same reading noise and dark current.