To study the flexural capacity of internal prestressed glulam beams, the relevant conclusions of experimental research at home and abroad were summarized, the methods to predict the failure mode and the formula of the flexural capacity were proposed based on Bazan's constitutive relation model, the plane-section assumption and the limit strain analysis method. The computation examples demonstrated that the calculated results according to the formula were in reasonable agreement with the experimental data. The relationship between the flexural capacity and the effective tensioning force was studied, and it showed that the effective tensioning force could determine the failure mode, and the mechanical property parameters of the compressive zone were the key influencing factors of the relationship. The internal force of the tendons under tensile failure mode or compression failure mode was approximately linear with the effective tensioning force. The upper and lower limit of the effective tensioning force were defined, which indicates that the tendons yield when the failure happens, and tension failure and compression failure occur simultaneously. The analytical solutions of the upper and lower limit were also provided.