Poly(ethylene terephthalate) (PET) ultrafine fibers were prepared by melt electrospinning technology, and the effects of main spinning parameters on the fibers diameter were described through orthogonal experiment. The results show that under the condition of this experiment device, the effective control of melt-electrospinning process can be realized, the diameters of the fiber present downward trend with increase of the spinning voltage; the fibers diameter gets thinner first and then gets thicker with increase of the accepted distance as well as the melt temperature; increasing in the feed pressure would increase the fibers diameter; improving spinning environmental temperature to glass transition temperature of the polymer, the average fibers diameter decreases significantly. The diameter of PET ultrafibers prepared under the condition of the optimized spinning process(spinning voltage 27 kV, accepted distance 7 cm, melt temperature 260 ℃, environmental temperature 70 ℃, fed by itself mass) is less than 1 μm on average.