The argon power cycle engine, which uses hydrogen as fuel, oxygen as oxidant, and argon other than nitrogen as the working fluid, is considered as a novel concept of zero-emission and high-efficiency system. Due to the extremely high in-cylinder temperature caused by the lower specific heat capacity of argon, the compression ratio of spark-ignition argon power cycle engine is limited by preignition or super-knock. Compression-ignition with direct-injection is one of the potential methods to overcome this challenge. Therefore, a detailed flammability limit of H2 under Ar-O2 atmosphere is essential for better understanding of stable autoignition in compression-ignition argon power cycle engines. In this study, with the HCCI code of CHEMKIN, the influence of argon dilution ratio, compression ratio, excess oxygen ratio, and engine speed on engine performance indicators including indicated thermal efficiency, CA10 (crank angle at 10% of total heat release), CA50, CA90-10, and in-cylinder pressure and temperature were studied. The results show that with a compression ratio greater than 12.5, H2 is flammable by compression-ignition in the argon dilution ratio range of 79%~99%. But with a compression ratio lower than 8.0, autoignition can be totally avoided. When the compression ratio is 11, the excess oxygen ratio is 4.0, and the argon dilution ratio is 81%, the indicated thermal efficiency can reach 55%. When the excess oxygen ratio is greater than 4.0, CA10 and CA50 will be ahead of top dead center, mainly due to the increased in-cylinder temperature caused by the increase in argon concentration. As the engine speed increases, CA10 is gradually delayed, which may compensate for the advanced CA10 caused by high excess oxygen ratio.