The frequency stability of a passive rubidium atomic frequency standard (RAFS) largely depends on the performance of the microwave cavity used in the physics package. In this paper, a large-size slotted tube microwave cavity with an inner diameter of 40 mm, as well as the physics package based on this cavity, were designed and built. Theoretical calculations and experimental results showed that the magnetic lines inside this cavity were highly parallel to the quantization axis. The frequency discrimination slope and the shot noise of the physics package were measured. The results showed that the cavity could be used to realize an RAFS with frequency stability of 2×10^-13τ^-1/2.