The invention relates to a method and a system for building a big data distributed log. The method comprises the following steps that: 1, a log transmission sub system receives log data from a service system, generates a UUID (Universally Unique Identifier) for each piece of received log data, and sends the log data with the UUID to a log storage sub system through a plurality of nodes after the load balance; 2, the log storage system receives the log data, and stores the log data in a horizontal expansion mode; and 3, a mass log processing sub system extracts the log data stored in the log storage sub system, uses a MapReduce algorithm to regularly perform mass pre-processing on the log data, and generates a report by the hour and a report by the day to be required by an external service report system. The method and the system have the advantage that the requirements on the reliability, the real-time performance, the high performance, the high expandability and the maintainability of a log system under the big data condition can be met.