With the development of the application of AlGaN/GaN high electron mobility transistors in the radio frequency field, a capacitance model that can accurately describe the C-V characteristics of the device has become an important research topic. The gate capacitance of GaN HEMT can be divided into two parts: intrinsic capacitance and fringing capacitance related to two-dimensional electronic gas (2DEG) electrode. The fringing capacitance plays an important part in the switching device. The outer fringing capacitance C_ofs/d dominates the fringing capacitance and is affected by the bias applied, especially the drain outer fringing capacitance C_ofd. In order to establish the C_ofd model which is related to the bias condition, the physics-based model of C_ofd is established based on the conformal mapping, including the drain channel length variable. Since the drain channel length is related to the bias applied, the channel length modulation effect can be used to study how bias apllied effect the channel, and the relationship between C_ofd and the bias condition is obtained. In addition, the threshold voltage variable is introduced when the channel length modulation effect is considered, and the threshold voltage drift caused by changes in the internal parameters and temperature of the device is studied using the threshold voltage variable in the model, and the relationship between C_ofd and threshold voltage and temperature under different bias was obtained. It is found from the results of the study that as drain bias increases from zero, the channel length modulation effect keeps C_ofd unchanged at lower drain bias. When the drain bias continues to increase, C_ofd begins to decay again, and its decay rate slows down with the increase of gate bias. The decrease of donor impurity concentration and Al component in AlGaN barrier layer may increase the threshold voltage, which will strengthen the channel length modulation effect on C_ofd, resulting in linear attenuation of C_ofd. With the increasing of drain bias, the influence of threshold voltage shift on C_ofd is enhanced, and the change of device operating temperature will enhance the threshold voltage shift and cause the deviation of C_ofd. Moreover, with the continuous increase of drain bias, C_ofd becomes more sensitive to the temperature variation.