The invention discloses a single-rod boiling critical test device and method of a nuclear reactor under the condition of six-degree-of-freedom motion. The test device can carry out a closed forced circulation and natural circulation flow heat transfer test, and includes a shield pump, a voltage stabilizer and an orifice flow rate. meter, preheating section, test section, heat exchanger, metal hose, make-up pump, etc. The test section adopts the method of electric heating, and the main circuit is placed on a two-ton six-degree-of-freedom motion platform as a whole, which can complete the relevant research on the influence of ocean conditions on the critical heat flux density. Deionized water is used as the flowing working medium, and the deionized water enters the horizontal serpentine preheating section from the stabilizer and the shield pump (natural circulation without the shield pump), then enters the test section, and then passes through the heat exchanger, and finally returns to the steady state. Pressurizer to form a closed loop.