•Polymer grouting trenchless repairing technique of drainage pipe is proposed.•Mechanical behaviour of drainage pipe repaired by polymer grouting is investigated.•The repaired effect of polymer grouting pipe is better than the concrete grouting. Polymer grouting technology is an economical and efficient trenchless way to repair leakage and settlement of drainage pipeline. In this paper, the mechanical behaviours of drainage pipeline under traffic load before and after polymer grouting and cement grouting trenchless repairing are investigated through three-dimensional (3D) finite element method (FEM). Four different working conditions, including normal pipeline, disengaging pipeline, polymer-repaired pipeline, and cement-repaired pipeline are considered. The effects of load type, load location, buried depth on the mechanics of pipe are discussed in detail. The results show that the traffic load has great impact on both sides of the load position within 4 m, and the dangerous points of the pipeline are located at the bell and spigot joints. The most unfavorable condition occurs when the location of the disengaging corresponding to the load position. The greater influence on the disengaging pipeline with deeper buried depth applied. The disengaging pipeline can be repairing effectively and reliably by both the polymer and the cement grouting technology. However, the polymer material is more valuable than the cement material in terms of the Mises stresses after repairing.