An auto flow steam trap has a hollow body, a partition panel mounted in the body, at least one float seat mounted on the partition panel and at least one float assembly mounted in the at least one float seat. Condensate flows through at least one condensate inlet formed on the body, the at least one float seat and at least one float assembly, and then is drained out of the steam trap from a transverse outlet formed on the body. No flash steam occurs when the condensate is drained out of the steam trap. With a non-return valve assembly of the float seat, the condensate and steam that have flowed out of the steam trap do not flow back into the steam trap so a float of the float assembly is not damaged.