Submarine landslides are a great hazard to offshore pipelines. The comparison and optimisation of pipeline routing schemes to reduce the potential submarine landslide risk is a key issue in offshore oil and gas development engineering. This paper presents an improved group decision-making evaluation method for offshore pipeline routing optimisation in areas prone to submarine landslides. An information integrity variable is proposed to adjust the relative weight of each factor considering the incompleteness of the engineering geological survey information and data. The credibility level of each expert involved in the evaluation, which is calculated based on the similarity and difference of the experts’ judgment matrices, is introduced to correct the information integrity variable, relative weights, and memberships. The group decision-making for offshore pipeline routing selection is then obtained based on the principle of the majority rule. Finally, a case of pipeline routing optimisation in the submarine canyon area of the Baiyun depression, northern South China Sea, is assessed by using the proposed method. The result shows that the proposed group decision-making method can enhance the objectivity of the assessment for the offshore pipeline routing optimisation under a subjective environment.