3~5 μm mid-infrared lasers have attracted much attentions due to their wide applications in defences, medicine, free-space optical communication and materials processing. A new scheme for obtaining a 3~5 μm flexible-wavelength, compact mid-infrared Raman fiber laser is proposed, using a 2.7~2.9 μm Erdoped fiber laser as Raman pump source and a ZBLAN fluoride fiber as Raman gain medium. Based on the nonlinear coupling equations of Raman fiber lasers, the 3~5 μm mid-infrared ZBLAN fiber 1st/2nd-order Raman lasers are numerically analyzed and optimized. The theoretical optimization is mainly focused on the effects of ZBLAN fiber length and output-mirror reflectivity on the Raman laser threshold and output power. The numerical results show: 1) the Raman laser threshold significantly reduces with the increase of the outputmirror reflectivity, and there always exists the optimized ZBLAN fiber length for the lowest Raman threshold (4.15 W); 2) in order to obtain the high-power Raman laser output, the optimized reflectivities of output mirror are 84~98% (1st-order) and 42~60% (2nd-order), the optimized ZBLAN fiber lengths are 6.7~8.9 m(1st-order) and 1.5~2.4 m (2nd-order), respectively. The optimized slope conversion efficiencies can be as high as 72.36% (1st-order) and 34.06% (2nd-order). These results could provide a theoretical guidance for such mid-infrared fiber Raman lasers.