The participation of amicus curiae in international investment arbitration can play an important role in assisting the arbitral tribunal in resolving the legal of factual issues in dispute. However, the amicus curiae assisting function provisions in international arbitration rules are of a principled nature and not operational, while there is insufficient interaction between the arbitral tribunal and the rules. A textual interpretation approach of the provision and an examination of judicial practice can clarify the elements inherent in the auxiliary function and guide the arbitral tribunal in determining the auxiliary role at a micro level. This paper suggests that it is advisable for the tribunal to specify the correspondence between the petitioner's concerns and the issues at stake of the case based on moderate approach, while aligning itself with the direction of arbitration reform - consistency of the award. Petitioners are suggested to grasp the intrinsic elements of the amicus curiae supporting function and characteristics of interpretation by the arbitral tribunal in order to make better use of the amicus curiae mechanism.