The invention provides a proportional steam trap with a floating ball automatic adjusting function, which relates to a device for automatically preventing steam and draining water. The steam trap consists of a splitter (1), a filter screen (2), a water-retaining board (3), a slag storage tank (4), a liquid storage chamber (5), a sealing cover (6), a float ball (7), a positioning rod (8), a connecting rod shaft (9), a connecting rod (10), a counterweight (11), a lever rocker arm (12), a lever rocker arm rotating shaft (13), a rotary valve core device (14), an air deflector (15), a guide tube (16), a non-return piece (17), an inlet pipe (18) and an outlet pipe (19). The steam trap has the advantages of automatic filtration, easiness of cleaning and maintenance, automatic adjustment of the floating ball, no noise when turned on or off, no mingled emission of gases and long service life.