The present invention provides a slag removing method of lime milk containing slag and includes the following steps (a) carrying through gravitational settlement slag removing separation on the lime milk containing slag to acquire the lime milk processed by the first step; wherein, the slag containing concentration of the lime milk containing slag is 3 to 5 percent of the weight calculated by the weight of the lime milk containing slag; (b) carrying through solid-liquid hydrocyclone separation on the lime milk processed by the first step to separate the solid slag from the lime milk processed by the first step to acquire the lime milk and solid waste slag processed by the second step with an average concentration of less than 0.5 percent of the weight; and (c) carrying through further filtrating and separating on the lime milk processed by the second step to acquire the lime milk after processed. The present invention also provides a device applied to the method.