An up to 8th order cascaded Raman random fiber laser with high spectral purity is achieved with the pumping of a narrow linewidth amplified spontaneous emission source. The spectral purity is over 90% for all the 8 Stokes orders. The highest output power is 6.9 W at 1691.6 nm with an optical conversion efficiency of 21% from 1062.0 nm. As a comparison, with conventional FBG-based fiber oscillator as pump source, only 47% spectral purity is achieved at 8th order. The temporal stability of the pump laser is proved to play a key role, because the time fluctuation of pump laser is transferred directly to Raman outputs and results in power distribution among different Stokes orders.