The present invention discloses a thermoplastic flame-retardant alloy and a method for preparing it, composed of the following components as below (parts by weight): 5-94 parts of ABS resin, 3-94 parts of polyester or co-polyester or the mixture of both, 1-50 parts of brominated flame retardant, 1-10 parts of flame-retardant aid, 0.001-30 parts of chlorinated polyethylene (CPE), 0.001-2 parts of anti-dripping agent and 0.1-6 parts of processing aid. Comparing with the ABS flame retardant presenting the prior art, the thermoplastic flame-retardant alloy features better glossiness, scratch resistance, stronger ultraviolet radiation resistance and better mechanical property, thus making up for the disadvantage that the flame-retardant ABS is prone to become yellow on some occasions with high weatherability requirements. Furthermore, the preparation method of the thermoplastic flame-retardant alloy for the present invention is successive, stable and feasible.