This study investigates the influence of truncation on the beam quality β factor measurement in Gaussian beams. Employing an annular energy approach, the paper analyzes the β method for calculating beam width's half-radius. Different truncation ratios of Gaussian beams are considered, deriving coefficients for precise β factor calculation. A critical truncation ratio is identified, beyond which diffraction effects are minimal, offering insights into beam measurement and production processes. This study provides key data and in-depth analysis for understanding and analyzing the beam quality β-factor of Gaussian beams induced by the truncation effect, thus laying the foundation for further research on the beam quality β-factor.