It is urgent at present to realize an all-weather space quantum communication and enhance its communication speed. The development history and the trend of quantum secure communication are summarized, and the development status and the research significance of the free space quantum key distribution (QKD) are also concluded. The source of bit errors and some existing problems of free space QKD are analyzed and the importance of background light suppression is discussed. The suppression methods are introduced and compared,and the next development direction of background light suppression is briefly discussed.