An auto flow steam trap has a hollow main casing having an inlet channel and an outlet channel, a float assembly mounted in the main casing, and a non-return valve assembly. With the non-return valve assembly mounted in the outlet channel of the main casing, the auto flow steam trap only needs one non-return valve assembly to control allowing the condensate to flow out of the main casing or not. Thus, the auto flow steam trap has advantages such as simple structure and low failure rate, easy assembling, and so on.