A donor-pi-acceptor (D-pi-A) alternative copolymer of carbazole and thieno[3,4b]-pyrazine [P(CZ-TPZ)] is synthesized through a Wittig-Horner reaction. In dilute THF solution, the absorption spectrum of P(CZ-TPZ) shows two absorption peaks at 306 and 452 nm, respectively, and the PL spectrum of the polymer solution displays a PL peak maximum at 543 nm. The polymer possesses relatively high sensitivity and selectivity for Hg2+ detection. Upon addition of Hg2+ into its THF solution (containing 0.3% CH3CN), P(CZ-TPZ) exhibits a new absorption peak at 560 similar to 600 nm and its emission was quenched dramatically. The Hg2+ detection shows high selectivity in comparison with the other cations of Na+, K+, Mg2+, Ba2+, Al3+, Cu2+, Cd2+, Pb2+, Ni2+, Mn2+, and Co2+. The Hg2+ detection limit of the polymer solution by emission quenching is found to be 1 x 10(-7) mol L-1. P(CZ-TPZ) also shows a selective chromogenic behavior toward Hg2+ with color change of the solution from yellow to blue dark which can be detected with the naked eye, the detection limit reaches 1 x 10(-6) mol L-1 with a 1 x 10(-4) mol L-1 polymer solution. The absorption and PL spectral change can be resumed after adding thiourea, therefore the sensing ability of the polymer is reusable with the treatment of thiourea. The results indicate that P(CZ-TPZ) is a promising chemosensor for the Hg2+ detection.