The mechanisms for improving the gain properties of the forward- and backward-pumped L-band double-pass erbium-doped fiber amplifiers (EDFA) are experimentally investigated. The gain enhancements of the two EDFA are compared for different EDF lengths. The results show that,the gain improvement mechanisms of the two pumping schemes are different. With the forward-pumped scheme,the population inversion near the signal input (output) port is very high,which helps to amplify the L-band input signal quickly after entering the EDFA and boosting its power before being exited. Such high L-band signal power near the signal input port may in turn suppress the backward C-band amplified spontaneous emission (ASE) near the input pump port,and thus,improve the L-band gain properties. With the backward-pumped scheme,the backward C-band ASE generated in the pump input port may be reused as an auxiliary pump source to enhance the L-band gain. However,for the backward-pumped scheme,the reuse of the backward C-band ASE may cause the gain self-saturation effect due to the high ASE level. It is found that the gain improvements of the EDFA with forward pumping scheme is slightly better than those with backward pumping when the EDF length is 20 m,because of the gain self-saturation effect caused by the high ASE. However,when the EDF length is more than enough for pump absorption,compared with the backward-pumped scheme,the forward-pumped L-band double-pass EDFA exhibits obviously insufficient pump,deteriorating its gain improvement,especially for the relatively low input signal power.