In Criminal procedure, expert witness is independent and unprejudicial. On the one hand, expert witness is entitled to know the case, to have access to relevant materials and equipment for the appraisal, to update knowledge, and to refuse unjustifiable demands; on the other hand, expert witness must fulfil the duties to accept legitimate demands of appointed appraisal and entrusted appraisal, to appraise strictly according to the facts impartialy and timely, and to serve as a witness at court as well when called upon. At present, the problems involving expert witness are becoming obvious in lawsuit practice. The qualification of expert witness mast be regulated by a system of verification.