An ion implanter and a method for implanting a wafer are provided, wherein the method includes the following steps. First, a wafer has at least a first portion requiring a first doping density and a second portion requiring a second doping density is provided. The first doping density is larger than the second doping density. Thereafter, the first portion is scanned by an ion beam with a first scanning parameter value, and the second portion is scanned by the ion beam with a second scanning parameter value. The first scanning parameter value can be a first scan velocity, and the second scanning parameter value can be a second scan velocity different than the first scan velocity. Alternatively, the first scanning parameter value can be a first beam current, and the second scanning parameter value can be a second beam current different than the first beam current.