In order to improve the demonstration experiment of light propagation along water flow based on the principle of total reflection of physical optical fiber communication in grade 9, the author came up with an innovative demonstration experimental instrument in light transmission of sound,which can visually and audibly present experimental phenomena intuitively;the author put forward several detailed self made schemes at the same time.There are three kinds of modulation amplification methods at the transmitter of experimental instrument: single power source common emitter amplification, resistive DC AC superposition after power amplifier, transformer boost amplification audio series DC source; The demodulation mode of the receiving end includes solar panels, photosensitive resistor partial voltage, infrared receiving LED.The paper also verifies the selection of the best combination of experimental instrument and briefly analyzes the causes of impaired sound quality.The experimental instrument can help students intuitively understand the principle of total reflection in optical fiber communication,and also show the interest of physics, the goodness of science, and the beauty of knowledge.