Physicochemical properties of wheat straw play crucial roles in bioenergy and chemical conversion processes. Four wheat straw cultivars at different maturity levels were collected in Beijing, China, and the growth days (GDs) ranged from 236 to 263 days. The physicochemical compositions and energy properties were analyzed. Cellulose (Cel), hemicelluloses (Hem), monosaccharidic composition of hemicelluloses (xylan, arabinan, hemicellulosic glucan, galactan, and mannan), and dry matter (DM) increased from 236 to 263 days, which can be explained by the increasing lignification of the cell walls in wheat straw with maturity. Water-soluble carbohydrates (WSC), crude protein (CP), nitrogen (N), phosphorus (P), and copper (Cu) decreased initially and then leveled off, with leaves decreasing. Lignin (Lig), ash, sulfur (S), potassium (K), and sodium (Na) increased first and then decreased slightly to 263 days. Carbon (C), hydrogen (H), oxygen (O), volatile matter (VM), fixed carbon (FC), and higher heating value (HHV) had litter variation with advancing maturity. A number of significant correlations were found among different physicochemical compositions. Regression equations for Cel and DM based on GDs showed excellent performance for prediction, while models for WSC and CP showed good prediction.