The high covertness of APT attack is an important feature that is different from traditional cyber-attacks, and it also can reflect the attacker's ability. The existing evaluation methods for the covertness of APT attack usually analyze the malwares used in the APT attack process. To evaluate the covertness of APT attack systematically, we propose a quantitative method. For multiple phases of APT attack, we construct two covertness metrics of each single attack phase considering the probability of the attack detection and the degree of disturbance to the target system, and then merge multiple phases to analyze the covertness of APT attack process. According to the results of simulation experiments, the method can evaluate the covertness of APT attack effectively and accurately. Quantifying APT attack covertness can help defenders to understand the specific process of APT attacks more clearly, and provides a method to learn about the ability of attackers. It also can give some significant suggestions for the distribution of defense resources and the defense strategies.