A wallmount dual beam sensing device has a microcontroller unit (MCU) mounted inside a housing and electrically connected to an ambient light detector, an infrared detector and a microwave sensing module. The MCU determines if the infrared detector and the microwave sensing module are activated according to detected luminance by the ambient light detector and outputs a control signal when the infrared detector and the microwave sensing module are activated and any one of the infrared detector and the microwave sensing module detects the presence of people in a monitored environment. Accordingly, the infrared detector and the microwave sensing module statically and dynamically sense the monitored environment in a 3D manner at the same time, thereby ensuring a stable and reliable sensing result.