The study on gas flow characteristics in a long-constant-area microchannel in mixed Kn-regime flows has not only theoretical meaning, but also important application in controlling the system of outer space aerocrafts. Flow characteristics were studied based on experiments and approximate theoretical analysis. The inlet pressure was 130, 250 and 320?kPa, and the outlet pressure ranged from 9 to 100?kPa. Five pressure measuring points were distributed along the microchannel and the temperature sensors were located at the inlet and outlet. The pressure distribution and the volume flow rate of air were measured experimentally. An approximate theoretical model based on Poiseuille flow was adopted. Experimental investigations with the long-constant-area microchannel indicate that the mass flow rate through the microchannel changes little as the inlet-to-outlet pressure ratio reaches a certain critical value. The phenomenon is defined as sub-choking and the corresponding pressure ratio is defined as the sub-choking-critical-pressure-ratio. The phenomenon of sub-choking is due to the surface effects. Moreover, the effects of the ratio of surface to volume on the critical pressure ratio are studied.