Abstract: To make the most of the accumulated multi-dimensional time series data，the paper studied the comprehensive evaluation index system and evaluation method to reflect the whole environmental quality of coastal cities． A comprehensive evaluation system was constructed from six elements of atmosphere，fresh water，sea water，noise，ecology and pollution discharge，which contained 13 underlying indexes． The scatter degree method was used for dynamic comprehensive evaluation to compare the environmental quality both vertically and horizontally． The method had the advantages of objectivity，less calculation，transparent process and high operability． The assessment result was unique without dispute． A case study was carried out in Dalian to verify the feasibility．