The invention discloses a pretreatment method for improving quality of tobacco stems. The pretreatment method comprises the following steps of: directly pressing tobacco stems subjected to leaf-threshing and redrying into flattened stems, adding an enzyme, and storing the flattened stems for 24 hours in an environment with the temperature of 40-50 DEG C and the humidity of 50-60 percent; and roasting the steam by using a roller at the temperature of between 140 and 150DEG C, controlling the water content of the flattened stems to be 11-13 percent, boxing for later use. By combining the conventional leaf-threshing and redrying and tobacco steam treatment process, a stem pressing procedure is brought forward, the tobacco stems subjected to the leaf-threshing and redrying are directly pressed, and structures of the tobacco stems are destroyed; the enzyme is adopted to act on the flattened stems, so that the enzyme can directly act on the internal structures of the tobacco stems, and the acting efficiency is ensured; and by matching a subsequent steam storing process, the inner quality of the tobacco stems is effectively improved, and the subsequent processing is facilitated. In the whole processing process, pollution is avoided, the conventional equipment is subjected to technology improvement to implement the method, and the method is high in operability and suitable for large-scale industrial application.