An optical detecting device capable of increasing signal-to-noise ratio (SNR) and economizing power consumption is installed on a wearable device. The optical detecting device includes a base, an optical detecting component and a light emitting module. The optical detecting component is disposed on the base and has a detecting surface normal vector. The light emitting module is disposed on the base and outputs a sampling signal to project onto an external object, and the optical detecting component can receive the sampling signal reflected from the external object. The light emitting module is slanted toward the optical detecting component, and an optical axis of spatial distribution of the sampling signal and the detecting surface normal vector are crossed to form a deviated angle.