As a new and practical technology of wastewater treatment, constructed wetland has become popular more and more. This paper introduced the purification mechanism of constructed wetland in detail, described the variable characteristics of oxygen in this technique and the removal mechanism of organism, nitrogen and phosphorus and characteristics of microorganism in wetland separately, and discussed briefly several primary factors that influence the purifying effect of constructed wetland. This work could provide the theoretical references for the popularization of this technology.