Visible light communication (VLC) has become a powerful and complementary technology to radio frequency (RF) communication thanks to its unrestricted bandwidth resources and high transmission rates. However, current VLC applications are limited to the short-ranged and interference-free scenarios because of the attenuation characteristics of light and the interference of ambient light. In order to extend VLC to various IoT applications, this paper tries to prolong the communication range of VLC systems based on M-frequency shift keying (MFSK), where denotes the modulation index. Specifically, 1) we modulate the luminous intensity of the LEDs to form sine waves instead of square waves and tune the sine wave frequency to transmit data. Compared to light intensity modulation, it can effectively resist the ambient light interference. Meanwhile, 2) we propose a demodulation method based on gradient thresholding to increase its fault tolerance in decoding. In addition, 3) we built a real VLC platform and conducted comprehensive experiments to verify the communication distance and reliability of the low-power VLC systems (with 1W/3W LEDs). The experimental results show that the proposed work successfully prolongs the communication distance to 5.8 meters with 1 W LED and to 10 meters with 3 W LED, extending the VLC range by nearly compared to existing work.