Cascaded all-fiber 1018 nm laser source is designed and demonstrated, which can boost the low input 1018 nm signal to 4.2 W. The main factors that have impact on the power conversion efficiency are analyzed. Then the tandem-pumped fiber amplifier is experimentally investigated in which the 1018 nm fiber laser is used as pump source to amplify the 1070 nm signal laser. In the core-pumping experiment, the signal laser is efficiently amplified with a high power conversion efficiency. The reason leading to low power conversion efficiency in cladding-pumped experiment is analyzed and some solutions are proposed.