High concentration Er3+-Yb3+-codoped fiber laser (EYFL) can provide high gain 1.54m laser emission. In this paper, simulation of the lasing operation of an ultra-short Er33+Yb3+-codoped phosphate fiber laser (EYPFL) was introduced using the theoretical models of high gain ultra-short EYFL with considering the energy transfer upconversion processes. The dependence of output power on fiber length, Er3+-Yb3+concentrations and output coupler reflectivity are investigated numerically. Based on the numerical analysis, when the pump power is below 800 mW, fiber length of 9.5-13 cm, Er3+concentration of 1.0-1.4×1026/m3and Yb3+concentration of 0.3-0.8×1027/m3are desirable to optimize the performances of EYPFL. Finally, the experimental results of a 11.6 cm EYPFL with the maximum output power of 230.4 mW is demonstrated in short straight cavity configuration with slope efficient of 45.7%.