The composites composed of polyamide66（PA66）, polypropylene（PP）, and magnesium oxysulfate whisker(MOS) were prepared by means of melt blending through co-rotation twin screw extruder. The standard samples were prepared by injection machine. The effect of coupling agents on the whisker surface, and the influence of MOS content on mechanical properties, heat properties, flame-retardant properties and the microstructure of the composites were investigated by scanning electron microscopy(SEM), thermogravimetry(TG) and differential scanning calorimetry(DSC) techniques, respectively. The results showed that the MOS whisker modified with coupling agents KH570 possesses better activation exponential and dispersity, and the tensile strength, flexural strength and toughness of PA66/PP/MOS composite were firstly increased and then decreased with the MOS content increasing, the optimum adding amount of MOS was 25phr. The flame retardant property was improved remarkable by adding of MOS.