In this article, a stealthy multiplicative attack design scheme for cyber‐physical systems equipped with parity space based detector is proposed, with the aid of gap metric method. First, a novel stealthiness index defined by the gap metric is presented for multiplicative attacks, and the security analysis issue of parity space based detector is formulated. Then, the attack subspace satisfying the stealthiness constraint is constructed, and the subspace rotation is applied to parameterize the multiplicative attacks. Moreover, a joint optimization method is presented to design the stealthy multiplicative attacks. Finally, simulations on a flight vehicle demonstrate the effectiveness of the proposed methods.