The invention provides a RSA private key mask operation method, a coprocessor and a RSA arithmetic device, wherein when a controller receives a start operation instruction sent by a master processor,the controller acquires a to-be-decrypted content m, a random number r, a public key e, a private key d and a modulus n of a RSA algorithm, and sends the decrypted content to the master processor; a modular adder calculates e-1 and calculates d-1; a modular exponentiation device calculates r<e-1>modn based on the e-1, calculates (mr<e>)<d-1>modn based on the mr<e>modn and the d-1; a modular multiplier calculates mr<e-1>modn based on the r<e-1>modn, calculates mr<e-1>.rmodn based on the mr<e-1>modn to obtain mr<e>modn, and calculates (mr<e>)<d-1>.mr<e-1>modn based on the (mr<e>)<d-1>modn and the mr<e-1>modn to obtain m<d>modn, and sends the decrypted content of the m<d>modn to the controller, thus, technical effects of reducing mask base cost, having relatively higher security in aspects, such as resisting side channel attacks and resisting fault attacks, are achieved.