As the rapid development of cities, foundation pit engineering develops quickly with more requirements to the realtime monitoring of foundation pit and its automatic alarm device. Based on AD7670and EP4CE6F17C8, this paper designs an automatic alarm device of foundation pit in order to avoid false or delayed alarm to dangerous condition caused by manual monitoring, calculation and alarm. The paper firstly make a general design scheme about automatic alarm device to foundation pit and then offer detailed introductions to its related hardware design, program and filtering algorithm. The results show that effective measurement to tilting value of foundation pit is achievable with errors within 5% basically satisfying technical specification of foundation pit engineering and the realtime alarm is workable for this automatic alarm device.