Polar codes are used in 5G system for the transmission of control channels due to its excellent error correction capability for short sequence, and CRC assistant successive cancellation list (CA-SCL) decoders are commonly used in practical system. When applied in critical environment, e.g. space platform, the memories in the hardware polar decoder will suffer single-event upsets (SEUs) that can cause failures and disrupt communications. Therefore, analyzing the reliability of polar decoder to SEUs on memories is important. This paper first analyzes the effects of single event upsets (SEUs) on the different parts of the memories in a CA-SCL decoder based on the decoding principle, including the influence of main parameters to the reliability of each part of user memory. Then fault injection simulations are performed to check the conclusions of theoretical analysis. Experiment results show that about 99% of SEUs on the memories can be tolerated by the decoder itself, and the reliability of polar decoders is higher for longer code, lower code rate and larger number of parallel decoder units. These results will be valuable to efficient protection of polar decoders to SEUs.