For a turbocharged diesel engine, the differential pressure in the intake port is up to 60~80kPa during the intake stroke, and the gas compressibility needs to be considered. Therefore, a formula of the discharge coefficient with consideration of the gas compressibility was theoretically derived and compared with the Ricardo’s formula. Furthermore, the steady flow simulation models of the direct intake port were established, and the effects of intake pressure and differential pressure on steady flow characteristics were studied. Results showed that when the compressibility of gas in the intake port became stronger, the calculation values of Ricardo’s formula were deviated from the true value much more, so the Ricardo’s formula cannot be used for the performance evaluation of the intake port in turbocharged diesel engines any more. However, the derived formula can be still applied to the flow capacity evaluation calculation. From the investigation of the steady flow characteristics, the influence of the compressibility on the air motion was irrelevant to the intake pressure, but it was relevant to r(differential pressure divided by the intake pressure), and with the increase of r, the influence was enhanced. When r was the same, with the increase of intake pressure, Caver (mean discharge coefficient) was increased. When intake pressure was the same, with the increase of r, Caver was firstly increased and then decreased. Thus under each intake pressure, with the variation of r, there was a peak, and the corresponding r was called rmax. With the increase of intake pressure, rmax was approximately decreased linearly.