Cold 87 Rb atom's loading in an integrating sphere with diffuse light is analyzed theoretically and experimentally. The experimental results show that diffuse light cooling has the greatest efficiency to cool the most atoms when the red detuning between the frequencies of cooling light and atom transition is about 3.3Γ (Γ is the natural linewidth, 6.065 MHz). Theoretical analysis using rate equation and numerical calculations on the cold atom number and loading time agree with the experimental results. This integrating sphere cooling would be a novel method for cooling atoms and lead to a new and robust cold atom source for atomic clock.OCIS codes: 020.0020, 260.0260.