Sludge pyrolysis and biomass gasification integrated process (SPBG) is an attractive route for the comprehensive utilization of the two materials but more tar is produced in this process compared to traditional biomass steam gasification. Nitrogen-containing compounds in the tar bring threatens to the environment and heavy components in the tar contributes to undesired coke formation. In current study, the evolution of heavy tar, especially the nitrogen-rich components, during SPBG is revealed for the first time. It was found that heavy components were mainly distributed in the mass range of 150-450 Da, where aromatics consisted of carbon, hydrogen and nitrogen atoms were the most abundant. Deamination (NH3) and the combination of quinoline accompanied with the generation of the heavy components. Organics from sludge could react with biomass to form heavier oxygen-containing molecules. Meanwhile, steam from sludge promoted heavy components to crack by tar reforming reactions and consumed radicals in bio-char to inhibit the catalytic cracking of tar. Under the combination of above reactions, more heavy molecules were generated at low sludge volatile/biomass ratio and the aromatic content in the heavy tar decreased at high sludge volatile/biomass ratio.