A pressure regulator has a main body, a piston, an elastic element and an adjusting element. The main body has an inlet and an outlet. The piston is mounted movably between the inlet and the outlet. The elastic element is mounted in the main body to prepossess that the piston to move toward the outlet of the main body. The adjusting element is mounted adjustably in the main body and selectively abuts against the piston to selectively close a flow path from the inlet to the outlet. Adjusting the position of the adjusting element results influences the compressed length of the elastic element. Therefore, the preset outlet pressure is fine-tuned.