Chaotic optical communication is a secure communication method which uses synchronous chaotic carrier to realize encryption and decryption. With the demand for information security increasing, chaotic encryption has been applied to space optical communication experimentally. However, due to the complexity of chaotic system design, the bit error rate (BER) of space chaotic optical communication system is much higher than that of traditional one. High BER limits the bitrate of space chaotic optical communication while bitrate is quite important since a large amount of information needs to be transmitted in space downlink. According to our previous work, introducing two diffractive optical elements (DOEs) into optical subsystem can improve transmission efficiency, which equals to increasing transmission power. In this manuscript, we further investigate the effect of DOEs on BER and propose some suggestions on BER enhancement of space downlink chaotic optical communication.