A method for optimizing CT scanning parameter is disclosed. A target group may be generated from a plurality of reference information samples. Each of the reference information samples may include subject information, information indicating a scanning protocol, one or more scanning parameter values and information indicating reconstructed image quality; the target group can consist of one or more reference information samples with the same subject information and the same scanning protocol. A scanning parameter optimization may be performed according to reconstructed image qualities and scanning parameter values of reference information samples in the target group, so as to acquire a target scanning parameter value of the target group. And according to the target scanning parameter value, a reference X-ray irradiation dose corresponding to the scanning protocol and the subject information of the target group may be determined.