A combined cooling solution is proposed to improve both thermal and energy performance for data centers with high heat density. Multi-stage heat pipe is introduced to make the internally cooled rack, which helps to illuminate the undesired mixing of hot and cold air, and makes a uniform distribution of indoor temperature. A water loop of multi cold sources is designed to make full use of waterside free cooling potentials. With the switchable and flexible operating mode, an energy efficient cooling can be expected. An operating data center in Beijing is studied and retrofitted using this solution. A comparative measurement is performed to validate the effectiveness of this combined cooling solution, which shows an improved indoor thermal environment and reduces annual cooling cost by approximate 46%. (C) 2015 Published by Elsevier B.V.