Herein, a one-pot microwave-assisted heating synthesis route was explored to prepare a g-C3N4 based composite photocatalyst, which involved just treating of a mixture consisting of dicyandiamide and multi-walled carbon nanotubes (CNTs) using a microwave oven. The composite photocatalyst prepared by this route exhibits closely contact interfaces between g-C3N4 and CNTs, as compared with the two ones obtained from the two-step and one-step methods previously reported, which lead to the reduced photoluminescence emission and longer life-time of the photogenerated carriers, and the increased photocurrent response. As a result, the hydrogen evolution and oxygen evolution rates of the WCN/CNTs composite are up to 45.1 mu mol/h and 7.14 mu mol/h, 10.02 and 4.61 times that of BCN, respectively, which are higher than those of the other two ones prepared by the previously reported methods. This one-pot microwave-assisted heating synthesis route is suitable for preparing the g-C3N4 based composite photocatalysts.