The invention provides an alarm system for preventing a child from being left in a car, and belongs to the technical field of child safety. The system comprises a human life detection part and an in-car environment monitoring part, wherein the human life detection part comprises a seat temperature monitoring part, a seat pressure monitoring part and a human thermal infrared monitoring part; the in-car environment monitoring part comprises an in-car temperature monitoring part and an in-car CO concentration monitoring part; monitoring data signals are input into a signal analysis and treatmentsystem by the human life detection part and the in-car environment monitoring part; after the monitoring data signals are analyzed by the signal analysis and treatment system, an alarm signal is sentout if a danger is found; when a child is in the car, and the temperature or the gas index exceeds a product design threshold, emergency treatment is triggered, an air-conditioning ventilation function is turned on, a sunroof is opened, the car whistle is triggered and a warning light flashes; at the same time, the alarm system is connected with a smart bracelet and an APP and triggers the alerting function of the bracelet and the APP; and when the dangerous environment is changed and the danger is removed, the alarm is stopped. Through the adoption of the system, the functions of performing timely emergency treatment and performing automatic alarming under the condition that a person is in the car and the environment inside the car is not suitable can be achieved.