Abstract: Aiming at the engineering problem that bored cast-in-place piles for high-rise buildings in karst areas of northern Guangdong require deep embedment into bedrock due to widely developed karst caves and dissolution fissures, which leads to excessive construction costs, this paper investigates the reinforcement performance of Cement Fly-ash Gravel (CFG) pile composite foundation on karst sites to make full use of overlying soil layers above bedrock and control project costs. Relying on a high-rise residential project adopting CFG pile composite foundation constructed by the long auger drilling and concrete pumping technology, field in-situ tests including low-strain integrity test, single pile vertical compressive static load test and composite foundation plate load test are carried out. The results show that CFG piles can greatly improve the bearing capacity of the foundation. The characteristic bearing capacity of natural foundation is only 120 kPa before reinforcement, while that of the composite foundation is increased to 320 kPa after treatment. The settlement develops gently under all load grades, and no abrupt inflection point appears on the Q-S curve under the maximum design load, indicating an adequate safety margin of bearing capacity for the composite foundation. CFG pile composite foundation is applicable to foundation reinforcement of high-rise buildings in karst areas of northern Guangdong with favorable reinforcement effects, which can provide references for similar projects on regional karst sites.