Here the core structure of a new variable optical attenuator (VOA) based on microfluidics is presented, where the microfluid and compressible air are used to continuously change the optical energy by adjusting the attenuator. Based on this core structure, two variable optical attenuators using different driving technologies are given. By the propagation theory of Gaussian beam, Helmholtz equation, mode-coupling theory and Mathematics software, the optical field and characteristics of this VOA are researched in theory and the diffraction effect is discussed. The theory results indicate this VOA has big attenuation range (50 dB), quick response (0.01 s), small insertion loss and strong return loss. The presented attenuator can provide a new method for development of small, integrated and adjustable optical communication devices.