1. A method for adjusting operating conditions of a fuel cell with an aid of a computing device including an empirical durability model generating unit, an optimal operation temperature estimating unit, and a memory including executable instructions stored thereon, the method comprising: obtaining a potential difference depending on a current density of a cell based on a thermodynamics reversible voltage, an activity loss, a resistance loss, and a concentration loss of a high temperature proton-exchange membrane fuel cell (PEMFC) including a polybenzimidazole (PBI) membrane with which phosphoric acid is doped (S 10 ); generating an empirical durability model which predicts a reduction in cell voltage over an operation time at a predetermined temperature (S 20 ) at the empirical durability model generating unit; determining an optimal operation temperature at a target life based on the potential difference depending on a current density of the PEMFC (S 10 ) and the durability model generated in the generating of the empirical durability model (S 20 ) (S 40 ) at the optimal operation temperature estimating unit; and adjusting an operation temperature of the PEMFC to the optimal operation temperature, wherein the generating of the durability model (S 20 ) includes: generating a draft model predicting the reduction in cell voltage over the operation time at the predetermined various temperatures while ruling out an effect of the operation time (S 21 ); performing a durability test of the PEMFC which detects the reduction in cell voltage for a predetermined period at the predetermined various temperatures; estimating coefficients of the draft model over various times generated in the generating of the draft model (S 21 ) based on the reduction in cell voltage detected in the durability test of the PEMFC (S 22 ); and generating a third-order non-linear function depending on the operation time and temperature using the coefficients estimated in the estimating of the coefficient (S 22 ) (S 23 ), and wherein for each of the predetermined various temperatures, the draft model in the generating of the draft model (S 21 ) predicts the reduction in cell voltage over the operation time using the draft model corresponding to the following Equation: 
 VD ( t )= x 1 t 3 −x 2 t 2 +x 3 t+x 4 , and a final model which is used at the durability model is to fit the four coefficients x 1 , x 2 , x 3 , and x 4 of the draft model depending on the operation temperature.