Abstract: As the fourth generation advanced reactor, the Brayton cycle system of small helium-xenon cooled reactor has the advantages of light weight, compact structure, simple system, fast start or stop, and is very suitable for the energy supply of remote areas and islands. Nuclear power plant is radioactive and highly hazardous after accidents, so it is very important to study its safety characteristics. In addition, helium-xenon cooled reactor is an advanced future reactor type, and its accident safety characteristics are not yet clear. Therefore, it is necessary to carry out the safety characteristics research of Brayton cycle system of small helium-xenon cooled reactor, especially under the condition of unprotected control. Firstly, based on Modelica language, through the modular dynamic modeling of key equipment, the accident safety characteristics analysis program of helium-xenon cooled reactor Brayton cycle system was developed. Secondly, the steady-state rated condition of the system was simulated, and the reliability of the accident safety characteristics analysis program of the system was preliminarily verified by comparing the error between the simulation value and the design value of the steady-state rated condition of the system, in which the maximum relative error is 2.31%. Finally, the transient conditions of various system accidents were simulated, and the dynamic response characteristics of transient conditions such as step introduction positive reactivity accident, step introduction negative reactivity accident, coolant incorrect charging accident and coolant incorrect discharging accident were analyzed. The results show that the simulation results of the system analysis program are reasonable. The reactor system has a certain self-stability and self-adjustment ability for step introduction positive reactivity accident and coolant incorrect charging accident under the condition of constant load and unprotected control. Under the conditions of step introduction positive reactivity accident and coolant incorrect charging accident, after the rotational speed increases by 21.6% and 12% respectively, it can be stabilized in another steady-state without protection and control. However, under the conditions of constant load and unprotected control, the reactor system can not be stabilized under another steady-state condition through self-regulation for the step introduction negative reactivity accident and the coolant incorrect discharge accident. Under the conditions of step introduction negative reactivity accident and coolant incorrect discharge accident, the corresponding rotational speed drop rate at 800 s is 85 rpm/s and 10 rpm/s respectively. It is necessary to control the rotational speed drop rate of the rotor through some protection and control measures to ensure the safety of reactor system. The relevant results provide a theoretical basis for the safety analysis of Brayton cycle system of helium-xenon cooled reactor.