An analytic equation of the mass flow rate through the bypass of double-inlet pulse tube refrigerators was obtained by an analysis of the pressure drop in the regenerator. The work loss through the bypass is discussed based on this equation. The work loss decreases with the increase in void volume ratio of the regenerator over the pulse tube, decreases with the increase in temperature ratio of the room temperature over the refrigeration temperature, and increases with the increase in “double-inlet factor, ” which is introduced in this paper. The work loss ratio at 80K with an ideal double-inlet is about 50, 30, 20 and 10% when the void volume ratio of the regenerator over the pulse tube is 0.5, 1.0, 2.0 and 4.0, respectively. The analytic results were compared with numerical results. A good agreement between them was achieved.