•The CIV and the DIV are innovatively proposed for index calculation.•This paper proposed the QCI based on EWM-FAHP and calibrated by FEM.•Damage correction function was used to measure the more deteriorating effect.•Suggestions can be put forward for the maintenance and repair of ballastless track by QCI. This paper aims to guide the maintenance and repair of ballastless track quality by determining a set of comprehensive evaluation indices. We take the China Railway Track System (CRTS) III slab track as an example and mainly look at two typical types of damage, namely, cracking and interlayer debonding. Based on the entropy weight method-fuzzy analytic hierarchy process (EWM-FAHP), cracking interspace volume (CIV) and debonding interspace volume (DIV) and subsequent weights were proposed to measure the damage degree. In addition, a model containing cracking and debonding was established using the ABAQUS finite element software, and these weights were modified based on the models' calculated results. Then, the quality condition index (QCI) was determined to assess the quality state of the slab track in a comprehensive manner. This index was further used in a case study to evaluate the quality of the slab tracks in real-scenario cases and thereby bring forward appropriate O&M suggestions.