In order to achieve the energy saving and management, this paper proposes an IoT-based energy-saving and management system for Telecom operators' remote computer rooms (RCRs). Firstly, a management system based on IoT technology for RCRs is designed. By using technologies such as embedded systems, sensor technology and Java Web, this system realizes equipment management and realtime data visualization of RCRs. Then, a simple but effective air conditioner energy-saving algorithm, called ET algorithm, is developed. Without causing high investment cost, ET algorithm minimizes the energy consumption of air conditioners which accounts for nearly 40% of total consumption, and thus improves the overall energy efficiency of RCRs. Moreover, the proposed system has been implemented at many Telecom operators' RCRs distributed at different regions in Guangdong Province of China, and stably operating for more than two years. Experimental results show that the proposed system not only effectively manages and monitors RCRs, but also achieves 16.64% energy-saving effect.