In recent years, the visible light positioning field has experienced remarkable advancements. However, smartphones find it difficult to identify light-emitting diode (LED) and extract each LED's light signal intensity due to the low-frequency and uneven sampling of built-in ambient light sensors (ALS, which is a photodiode that measures ambient light in lux units). Thus, traditional visible light positioning systems cannot be directly applied to smartphones. In this Letter, we propose a single-light visible light positioning system using a non-modulated LED as an emitter, the built-in ALS as the receiver, and the inertial measurement unit of the smartphone to assist in measuring the smartphone's attitude. It only requires the user to turn the smartphone by a few angles in a stationary position to estimate its current three-dimensional (3D) spatial position. This method does not require modification of the existing lighting system and consumes less power than the camera-based visible light positioning (VLP) systems. We have built an experimental site measuring 5 m × 5 m × 2.2 m to evaluate the performance of the positioning system, and the preliminary results show that the proposed system achieves sub-meter-level positioning accuracy.