A trace gas sensor based on all optical quartz-enhanced photoacoustic spectroscopy (QEPAS) is designed and demonstrated. An extra detection light beam is added to the traditional QEPAS setup to convert the amplitude of vibration of the quartz tuning fork, which is proportional to the concentration of the target gas, into the variation of the detection light intensity. As a result, there are not any electrical components near the measurement position. Such a design makes it immune to electromagnetic interference and provides a compact sensor head, which can be used in a tiny space or an adverse environment. Using the new setup to measure the water concentration in air, the obtained noise equivalent absorption coefficient is 1.13×10-6 cm-1W/√Hz. The approaches to further optimizing the system and improving the sensitivity are also discussed in detail.