In this work, we have synthesized a novel hemicyanine-modified polyacrylonitrile fiber (HDBA-PAN(A)F) as a colorimetric Hg2+ sensor. The color is observed from dark red to orange by the naked eye only when the fiber is exposed to Hg2+. Positively, the HDBA-PAN(A)F shows high selectivity and sensitivity for Hg2+ compared with other metal ions such as Na+, Mg2+, Al3+, Ca2+, Cr3+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, Cd2+ and Pb2+. Furthermore, the obtained fiber has a wide pH application range from 3 to 11 and also processes a lower detection limit of 1 x 10(-6) mol L-1 for Hg2+. Moreover, the HDBA-PAN(A)F undergoes an obvious color change toward Hg2+ even containing an excess of ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) in the solution, indicating that the fiber has a stronger complexation affinity for Hg2+ than EDTA-2Na. Hence, the fiber sensor possesses the merits of low cost, high selectivity, low detection limit and environment-friendly property, which shows that it is a promising candidate for Hg2+ detection in water.