The study of mixed convection in a horizontal channel with a free upper surface is not only able to guide the design and optimisation of many industrial applications, such as heat exchangers, evaporative cooling devices, but also beneficial for further understanding the nature of liquid flowing instability. By using the software of FLUENT, this paper presents a numerical study of laminar mixed convection in a horizontal channel with a uniformly heating bottom and a free upper surface where fluid convects with air. The temperature of sidewalls are both constant. The results showed that in the range of the dimensionless numbers we studied, the secondary flow of liquid is in the form of longitudinal rolls. The rolls appear near the sidewalls of the channel. As the liquid flows, the rolls gradually appear in the centre of the channel. The occurrence of longitudinal rolls improves the local heat transfer coefficient. The increase of Reynolds number lengthens the entrance region, and also strengthens the overall heat transfer. The variation of Biot number has almost no effect on the heat transfer near the sidewalls, but in the central region, the local Nusselt number increases with Biot number.