Urban development information and observed atmospheric temperature over the past 28 years are analyzed to study the relationship between urban cluster development indexes and urban heat island effect in the Pearl River Delta Region. The results reveal that urban cluster development indexes increase slowly before 2000, but soar five to nine times afterwards. Warming rate of the urban heat island intensity (UHII) is 0.34 ℃·(10 a)~(-1) before 2000, and doubles to 0.69 ℃·(10 a)~(-1) after 2000. The UHII increases year by year, and changes significantly during a year. It is weak in rainy seasons, but strong in dry seasons. In July when rainfall is abundant, the UHII is the weakest, whereas in December when the weather is dry, it is the strongest. Gray relational analysis indicates that, urban development indexes, including city built-up area, gross industrial output value, total annual electricity consumption and resident population significantly affect atmospheric temperature and the UHII. Urban development indexes are sorted according to their correlation coefficients with atmospheric temperature and the UHII in descending order. Then, indexes with greater correlation are selected to build a gray model, simulating atmospheric temperature and the UHII. This gray model performs well with the mean relative fitting error of urban cluster development indexes being 2.7% and 7% to air temperature and the UHII, respectively.