Abstract: In order to evaluate the working performance of fluidized sand biofilter, and determine the optimal design parameter, a swirling fluidized-sand was designed, which used quartz sand as media. This paper discussed the effect of size and initial thickness of sand bed on the fluidizing performance. Double factor test was designed with different ranges of quartz sand (0.18 to 0.25, >0.25 to 0.425 and >0.425 to 0.6 mm) and different initial thickness of sand bed (40, 50, 60, 70, 80 and 90 cm). The results showed that the minimum fluidization velocities in three groups of different sizes range for quartz were (0.061±0.0088) cm/s, (0.25±0.011) and (0.48±0.014) cm/s, respectively. To keeping sand bed at optimum fluidized state, the ratios of initial thickness of sand bed to the diameter of circular vessel should be greater than 1.43, 1.78 and 2.14, respectively, and the ratio was proportional to the size of quartz sand. The superficial velocity remained constant, while the initial thickness of sand bed increased at the same expansion ratio state. The pressure drop test showed that the pressure drop values per unit sand bed thickness for different sizes of quartz sand were (7530.66±215.98), (6925.66±364.58) and (6790.08±277.95) Pa/m, respectively. The measured value was close to the theoretical value under the condition of quartz sand with 0.18 to 0.25 mm, and error was 2% to 3%. Based on Ergun equation, the mathematical model of minimum fluidization velocity was proposed according to the measured value by regression method, and it can provide a technical basis for the design and application of fluidized-sand biofilter.