In order to analyze the influence of atmospheric turbulence on the performance of free space Quantum Key Distribution (QKD), the turbulence is modeled as randomly distributed atmospheric bubbles so as to conceive the propagation process of single photon using geometric optics. The fluctuation of photon transmission ratio k and Quantum Bit Error Rate (QBER) Ep has been quantitatively calculated, which both are caused by the change of polarization state after continuous refractions. Then the trends of these two values are simulated with random turbulence refractive index using Monte Carlo method. Furthermore, the key generation rate of decoy state free space QKD has been derived under turbulence situation. The upper bound of k is achieved through analyzing the source of QBER and the relationship between Ep and incident angle and turbulence refractive index is established which defines the secure threshold of Ep. Simulation results show that when incident angle is between 44.8° and 76.5° while the refractive index falls within 1~1.33 will meet the required upper bound of Ep, which provides theoretical reference to quantum communication in turbulence scenario.