The last stage of a He-4 Joule-Thomson cryocooler, which is composed of a recuperator, a JT valve, and a cold heat exchanger, is investigated. It is found that, for given precooling temperature and high and low pressures, the JTC can work steadily with any heat load less than a certain maximum. When the heat load exceeds this maximum the temperature of the last stage rises without limitation. This situation is defined as the overloaded condition. With pressure-enthalpy diagram analysis, the temperature behavior is explained theoretically and verified by the experimental results. The mass flow change along with the upstream state of the JT valve is analysed with the theory of the two-phase choking. (C) 2017 Elsevier Ltd. All rights reserved.