An adjustable net post assembly used in ball games comprises: an inner sleeve, an outer sleeve, a spring, an outer pipe and an inner pipe. By rotating the outer sleeve, the protrusion of the outer sleeve is moved to the first positioning portion or the second positioning portion the slot of the inner sleeve, so that the positioning protrusion of the positioning member will be disengaged from or engaged in the positioning hole of the inner pipe, which allows the inner pipe to be adjusted by being retracted or extended with respect to the outer pipe.