Photonic crystal superprism is the key device for the light deflection, filtering and so on, and the group delay is one of the most important parameters to judge light deflection and filtering performances. For the sake of achieving large shift separation, cascade Fabry-Pérot filters or Gires-Tournois filters are usually used to achieve large and linear group delay. A novel method for calculating group delay of multi-cavities filters based on multi-mirror theory is presented, which simplifies the calculation and is not restricted by the practicable materials. As an example, the group delay of a four-cavity Gires-Tournois filters is obtained, which is in excellent agreement with the result by transmission matrix method in optical thin film theory and proves its validity. Finally, the relationship between group delay, linear region and cavity length of Gires-Tournois filters is analyzed, and the suggestion on how to choose suitable cavity length is presented.