<p>To explore a new type of steel–concrete composite beam, which can save the material to reduce the self-weight of the members, and also facilitate the arrangement of the pipeline. In this paper, a partially encased steel–concrete composite beam (PECB) with web opening is proposed. This new composite beam has several through geometric holes in the web that can be used for piping facilities to pass through. A total of six PECBs with web opening and one comparison beam were tested in flexural testing. The effects of flange thickness, opening shape and opening ratio were mainly considered. The crack development was summarized and the relationship between moment deflection, rotation angle and strain were analyzed. The web openings were observed to cause stress concentration and vertical cracks were mostly concentrated in the mid-span. The greater the number of sides of the opening shape (positive polygon), the smaller the effect on the flexural capacity of the beam. The increasing ratio of openings reduces the flexural capacity of the beam. Finally, in this paper, the prediction models of cracking moment, initial flexural stiffness and ultimate flexural strength are developed based on the idealized cross-section stress and the discounted mean stiffness method. By comparing the predicted values with the tested values, it was found that the prediction error of the model was within 10%. However, the predicted values of ultimate flexural strength are mostly larger than the tested values, and a large amount of data is needed for further correction to better guiding practice.</p>