PbS colloidal quantum dots (QDs) have emerged as one of the most promising photovoltaic solar cells material. The synthesis of PbS QDs generally uses TMS2S as sulfur source. However, the volatile and environmentally unfriendly properties of TMS2S will limit its industrial applications. This paper presents a new method of synthesizing PbS QDs employing a CdS QDs/ODE solution as the sulfur precursor by exploiting a cation exchange reaction. The resulting QDs show a performance comparable with previous work for solar cells. Through optimizing the architecture of the photoactive layer, excellent stability in air is achieved. An efficiency of 7.89% is retained in air after 110 days.