In this paper, the 795-nm vertical cavity surface emitting laser (VCSEL) with sub-wavelength grating coupled cavity is proposed and designed based on the theory of resonant coupled cavity, and the mechanism of multi-cavity coupling linewidth narrowing and influencing factors are analyzed in detail by using the COMSOL software finite element method. The analysis results show that when photonic resonance takes place in a multi-coupled cavity, the grating-coupled cavity with reasonable design parameters and the multi-coupled cavity formed by precisely controlled lasers are phase-matched, which greatly strengthens the narrowing effect of the spectral linewidth resonance, and a 795-nm VCSEL laser with high beam quality and ultra-narrow linewidth output is obtained, finally. Theoretical results display that the reflection spectrum cold cavity linewidth Δλc of the coupling cavity with a thickness of 180nm of the spacer layer can reach 7 pm, which lays a theoretical foundation for achieving a kHz-level spectral linewidth output of VCSEL lasers.