Manhole cover monitoring devices are urgently needed in underground pipe network construction. Because the monitoring device is installed under the manhole cover, three key problems, how to measure the tilt angle or state of manhole cover, transmit data from under the manhole cover, and operate in low power for long service life, are researched and tested. For the first problem, the three-axis digital accelerometer is used. The angle algorithm with clear physical concept and small amount of computation is proposed. It is suitable for monitoring device with limited resources. The resolution of the tilt angle measurement is 0.01° and the maximum output linearity error of monitoring device is 0.63°. For the second problem, long range (LoRa) is adopted. A 433-MHz whip antenna is designed to overcome the shield of manhole cover and the absorption of electromagnetic waves of the earth. The field tests show that the effective communication distance has been extended more than 700 m by using the whip antenna. In addition, the parameter spreading factor (SF) and bandwidth (BW) are configured. For the third problem, sleep mode is used and input output (IO) pin is configured. Power supply stable circuit ensures successful communication of LoRa. Monitoring devices have been developed and tested in field. Long-term tests have been going on for more than two years. The monitoring device can meet the requirements of accuracy of measurement and communication distance for manhole cover state monitoring. Low-power design can extend the service time.