The security issues are of prime importance for Cyber-Physical Systems (CPSs). Most existing works mainly investigate the secure control schemes against malicious attackers. This article analyzes how to design an data integrity attack scheme from the viewpoint of an attacker. The formulation of our approach is basically similar to the one of conventional optimal control method, with different prerequisites and solutions. An output feedback control system under data integrity attacks on actuators is considered in this article. Our work is aimed at constructing an optimal feedback attack law to maximize the error between the attacked system's output and the healthy system's output. Numerical examples are presented to demonstrate the effectiveness of the proposed method.