The 3D electromagnetic and RF circuit co-simulation approach is a fast and accurate tool to calculate the EM fields of RF coils. It is typically used for human coils to evaluate the transmit field and SAR. In this work, we apply this method to the circuit optimization of a small animal coil. Unlike human coils, the coil noise of small animal coil is not neglectable and should be minimized. With the guide of co-simulation approach, an optimized saddle-shaped surface coil has considerable transmit efficiency and SNR improvement on ex-vivo squirrel monkey brain imaging at 9.4T.