Based on energy conservation and emission reduction of the 12th Five-Year Plan, for the purpose of realizing effective energy diagnosis and audit based on big electricity users, a rapid energy audit method combining with hardware and software is proposed and one low-side and mid-side energy-saving diagnosis instrument based on double AVR MCU and DSP2812 respectively is designed, meanwhile, supporting analysis software is developed, which constitute a complete intelligent system. The hardware monitors and records users’ energy consumption situations which provide basic data supports, the software uses the data to make deep diagnosis analysis. According to different types of users, the system can realize power quality monitoring online, energy-saving potential analysis, and rapid energy audit. Hardware and software design are introduced, and the results of field test are presented to demonstrate its feasibility, thus can realize the goals mentioned above. Its advantages include strong intelligence, low cost, highly reliable, easy-to-use, thus can effectively improve the efficiency, information degree and automation degree of energy audit work based on big electricity users.