The invention discloses a waste heat utilization process for rotary kilns. The process includes the steps of firstly, preheating tail gas of a body of a rotary kiln by a preheater, removing dust, purifying, subjecting purified hot gas to oxygen content detection by an oxygen detector, and entering a second step when oxygen content is lower than a preset value; allowing an alarm to warn when the oxygen content reaches the preset value, and automatically supplying and adding nitrogen till the oxygen content is lower than a preset value; secondly, supplying the hot gas to a drying coal mill system, and feeding pulverized coal into a pulverized coal bunker; thirdly, conveying the pulverized coal to a rotary kiln combustion system through a special pipeline to allow waste heat utilization. According to the process, waste heat utilization and waste gas treatment are combined, the online real-time detection and alarm device is added, waste heat, waste gas and oxygen content used by the system are detected continuously online, low-oxygen operation is guaranteed, and the system is safeguarded.