L-band extended erbium-doped fiber amplifiers (LE-EDFA) have been studied in recent years for their extended gain bandwidths. However, the pump conversion efficiency and gain level are generally low due to the relatively long gain fiber required for this amplifier. In this paper, we improve the gain level of LE-EDFA by employing the backward amplified spontaneous emission (ASE) co-pumping technique. It is found that gain level improvement up to 5.1 dB?16.7 dB can be achieved in the wavelength range of 1570?1620 nm when the pump power is relatedly low or the Er/Yb co-doped fiber (EYDF) is relatively long. We then use the backward ASE co-pumping technique in a pre-amplified LE-EDFA and achieve a 20 dB gain bandwidth covering 1570?1610 nm and noise figure less than 6.0 dB at a relatively low pump power of 550 mW. And the gain at 1620 nm is 13.7 dB. Compared with other reported LE-EDFAs, the L-band extended EDFA co-pumped by backward ASE achieves the same high gain level at the lowest pump power. The extended gain bandwidth, high pump conversion efficiency and low cost make it very prospective in high-capacity optical transmission systems.