This article mainly investigated the length distributions of the alkali-free short glass fibers in specimens and their effect on the mechanical and thermal properties of the composites. The results show that the initial length, addition level and feed way of the fibers have obvious effects on the length distributions of fibers in specimens, and thereby the mechanical and thermal properties of the composites. The main-direction feed way has an intense shear action on the fibers in specimens. With the increase of the fiber content, the reinforcing effect of fibers on the tensile strength, flexural strength and flexural modulus of the composites is increased, while the impact strength is decreased first and then tends to be stable, and the strength factor (F) of the tensile strength to weld line is significantly reduced. The longer the fiber lengths in specimens are, the more obvious the reinforcing and toughening effects are. To some extent, with the increase of the fiber content, the storage modulus (E 0 ) and loss modulus (E 00 ) of the specimens are increased, but the loss factor (Tan d) is reduced. The effect of the fiber initial lengths on the heat-degradation of composites is smaller than that of the fiber content. Meanwhile, adding fibers can improve the thermal stability of the composites, and this law is also confirmed by the heat deflection temperature (HDT) test.The blends dominated mainly by PC (polycarbonate) and ABS (acrylonitrile-butadiene-styrene) are important engineering materials, which have good molding flow properties, high HDT and thermal stability, excellent comprehensive mechanical properties and a certain flame retardancy. PC/ABS blends have been widely applied in automobile, appliance, electrical apparatus, defense industry and others. 11-17 PC/ABS melt presents a pseudo-plastic flow characteristic of the shear-thinning and their composites show a strong nonlinear behavior under the impact and compression. 17,18 Meanwhile, although there is certain compatibility between PC and ABS, the further modification of compatibility is still needed to obtain higher performance of PC/ABS blends. The related studies have found that some additives (EVA-g-MAH, SMA, etc.) can improve the compatibility between PC and ABS, thereby improve the impact performance and elongation of materials. 18,19 The researches from Din and Hashemi 2 and Hashemi and coworkers. 20-23 showed that when the reinforcing fibers are added into composites, the tensile strength, flexural strength, tensile modulus, and flexural modulus of the plastic parts are improved at different degrees, and elongation, work of fracture are decreased. Meanwhile, the weld line strength factor will be