The invention discloses a method for forming a pipe fitting with internal high pressure and external low pressure. The method comprises the steps that a pipe fitting to be formed is put into a cavity,a swelling region is formed between the pipe fitting to be formed and the inner wall of the cavity, low pressure fluid is continuously injected into the swelling region, then high pressure fluid is injected into the inner part of the pipe fitting to be formed, the high pressure fluid forms the pressure on the outer wall of the pipe fitting to be formed, so that the pipe fitting to be formed expands to the swelling region, when the pipe fitting expands to fit the inner wall of the cavity, supplying of the low pressure fluid is stopped, the formed pie fitting is taken out, and a working cycle is completed. The invention further discloses a forming machine, and the pipe fitting is formed according to the forming machine for a pipe fitting with internal high pressure and external low pressure. According to the method for forming a pipe fitting with internal high pressure and external low pressure, a first through hole and a second through hole are formed in a die for circulating the low pressure fluid into and out of the swelling region, a certain pressure P1 is formed for pipe fitting, so that it is guaranteed that the pipe fitting can swell uniformly everywhere, and a good forming effect is achieved.