The invention relates to a preparation method of low-cost composite polyurethane elastic fibers. The method comprises the steps that low-cost thermoplastic elastomers and inorganic powder are selectedas physical fillers of the polyurethane fibers; the inorganic powder is modified by cellulose ether substances; the thermoplastic elastomers, the inorganic powder and polyurethane are uniformly mixedto obtain composite spinning dope by adopting a solution blending mode; the composite polyurethane elastic fibers are obtained from the dope through an ordinary dry spinning technique. The mechanicalstrength and resilience of the polyurethane fibers made by the technical method are well maintained, and the economic cost can be reduced by 20-40% compared with that of ordinary polyurethane fibers.The elastic fibers are particularly suitable for the fields of webbing, elastic bands, diaper waistbands and the like.