Currently, Internet of Things (IoT) as an essential infrastructure proposed for industries and different applications has been popularly applied to different domains, such as healthcare and smart farming, for helping people to do something, aiming to improve our living environments. LoRaWAN, as a Long-Range Wide Area Network specification recommended by the LoRa Alliance, is a low power and long distance communication protocol suitable for IoT environments. This protocol adopts a widely used data encryption method, i.e., Advanced Encryption Standard (AES), developed based on powerful algebra operations and multiple encryption cycles to ensure its communication security. LoRaWAN reduces communication power by setting different transmission latencies for different end-devices; however, AES does not take into account its end device's encryption power. In this paper, a high secure but low-power consumption communication scheme for the LoRaWAN, named the Secure Low Power Communication (SeLPC) method, is proposed to further reduce end-devices' data encryption power by reducing encryption cycles of AES. In the SeLPC, encryption key and D-Box update procedure is presented to enhance security level and simplify the AES encryption process so that the power consumption can be further lowered. Comparing with the traditional AES, the analysis results show that the SeLPC can minimize the encryption power up to 26.2%. The SeLPC can also resist three attacks, including known-key, replay, and eavesdropping attacks and is practically helpful for use in LoRaWAN IoT environments.