The resulting slag particles from solid rocket motor( SRM) firings are an important component of space debris environment. Slag sizes as large as 1 cm have been witnessed in ground tests,and comparable sizes have been also estimated via observations of sub-orbital tail-off events. We achieve slag initial data based on MASTER slag model and SRM historical launch data,and propagate slag long-term orbital evolution taking into account the zonal harmonics J2,atmospheric drag,solar radiation pressure and luni-solar attraction to discuss the slag size distribution and orbital characteristics. Finally,future slag debris environment is evaluated based on two different launch rate assumptions. The result shows that current launch frequency will make the slag population sustain growth and the population will not decrease at once even if there are no more launches in the future. The resulting slag particles from solid rocket motor (SRM) firings are an important component of space debris environment. Slag sizes as large as 1 cm have been witnessed in ground tests, and comparable sizes have been also estimated via observations of sub-orbital tail- off events. We achieve slag initial data based on MASTER slag model and SRM historical launch data, and propagate slag long-term orbital evolution taking into account the zonal harmonics J2, atmospheric drag, solar radiation pressure and luni-solar attraction to discuss the slag size distribution and orbital characteristics. Finally, future slag debris environment is evaluated based on two different launch rate assumptions. The result shows that current launch frequency will make the slag population sustain growth and the population will not decrease at once even if there are no more launches in the future.