The behaviour of the various gas elements which enter the tube of a pulse tube refrigerator from its cold end has been analysed using the method of characteristics. It is found that, in an orifice pulse tube refrigerator, the gas elements can be divided into three parts. The specific cooling capacity produced by the second part of the gas elements is the largest. If the total mass is fixed, in order to improve the overall cooling capacity of an orifice pulse tube refrigerator, the ratio of the gas elements in the second part should be increased, while those in the first part and the third part should be decreased.