Film cooling characteristics and mixing loss mechanism changed by the mainstream Mach number (Ma_m) were numerically simulated for double-jet hole and contracted double-jet hole. The results showed that the lateral average cooling effectiveness near the air film hole area is promoted and the film is closer to the wall when Ma_m increases. The core of coolant of the two structures are mainly composed of the coolant of the downstream film hole. The coolant from the upstream hole makes the coolant from the downstream hole closer to the wall, and the anti-kidney vortex pair formed by mixing between the mainstream and coolant can prevents the film lift-off. In terms of mixing loss, the velocity and temperature difference between mainstream and coolant increases with the increase of Ma_m, and the entropy caused by the film cooling increases. The entropy caused by velocity difference mainly occurs near the outlet of the downstream film hole; the entropy caused by the temperature difference occurs near the mixing interface between the main and film.