In order to obtain the relationships between cloud surface long-wave radiative forcing and cloud, the effects of cloud amount, cloud base height and cloud optical depth on cloud surface long-wave radiative forcing are investigated by using SBDART. And the causes of the nonlinearity of the relationships between cloud surface long-wave radiative forcing and cloud amount derived from other experts are analyzed. The results show that, the cloud surface long-wave radiative forcing of the same cloud shows different characteristics in different atmospheric conditions. The cloud surface long-wave radiative forcing increases with cloud cover, and decreases with cloud base height. The cloud surface long-wave radiative forcing also increases with cloud optical depth, but the increasing stops when the optical depth is greater than 10. The nonlinearity of the relationship is caused by the more frequent occurrence of low altitude cloud and cloud of great optical depth as the cloud cover increases.