We report that carbon honeycomb, a new three-dimension carbon allotrope, exhibits large negative Poisson's ratio, as large as -0.32, in tensile revealed via molecular dynamics simulations. The Poisson's ratio of carbon honeycomb is anisotropic, and sensitive to temperature. The carbon honeycomb has phase transformation from normal to auxetic by tensile, along both zigzag and armchair directions. The critical strain for the normal-auxetic transition along the cell-axis direction reduces with respect to an increase in temperature. Combined with high strength of 50 GPa, such a unique and adjustable negative Poisson ratio suggests broad engineering applications of carbon honeycomb.