The theory of population rate equations was used to investigate the kinetics of upconversion ultraviolet laser based on the highest level 1S0 of 4f configuration in Pr3+:ZBLAN fiber. 586 nm and 335 nm pumping lasers are used to populate 1S0 level through stepwise two-photon absorption, and 250 nm ultraviolet (UV) continuous wave (CW) laser can be achieved by laser transition from 1S0 to 3F4. The threshold pump powers, output power, slope efficiency and optimum cavity length are calculated. The results show that the threshold pump power increases with the fiber length, and slope efficiency decreases with the increase of fiber length. Higher output power cannot be obtained in a short fiber. Moreover, higher doping concentration makes laser oscillate with lower threshold pump powers but higher slope efficiency. The best slope efficiency is 16.7% in the conditions taken in this paper.