As the penetration of photovoltaic（PV） generation increases rapidly in recent years， capacity credit of PV is becoming one of the hot topics in power system planning. However， the optimum orientation and tilted angel of grid connected fixed PV arrays are still obtained according to the principle of maximum energy value， which limits the comprehensive value of PV generation. The power output model of PV arrays is established first， followed by the effects of orientation and tilted angel on energy value and capacity value analysis. Finally， a novel optimization method for orientation and tilted angel according to the principle of maximum comprehensive value is proposed， and its effectiveness is proved by tests in a simulation system.