To research the principle of safe-error attack and RSA algorithm, the modular measures based on the hardware modular multiplier is taken as the analyzed target. Propose a fault analysis on RSA using SPA-FA resistant. The attacker inject fault into one of the multipliers during the modular exponentiation, and then it can use the output to deduce the corresponding bits of key. The vulnerability of RSA using SPA-FA resistant based on hardware modular multiplier is proved in theory, the practicability of the attack is analyzed in three aspects. The result shows that RSA algorithm can provide effective resistant measures safety and research with reference when there is safety leak in hardware multiplication situation.