Plant fiber reinforced cementitious composites (PFRCC) panel has become the focus of current research due to their advantages of being lightweight, high strength, and good machinability. However, most scholars have studied the effects of different kinds of plant fibers on their mechanical properties, and few scholars have studied the effects of thickness changes on their mechanical properties. Therefore, a new plant fiber reinforced cementitious composites panel was developed in this paper, and the influence of thickness change on its mechanical properties was studied by axial tensile test and three-point bending test. The results show that: (1) With the increase of the thickness of the PFRCC panel, the tensile failure load increases gradually, but the peak tensile strain and tensile strength remain unchanged. The peak tensile strain is about 0.022 %, which is 1.47-2.2 times that of the matrix cement mortar. The average tensile strength is about 2.52 MPa, which is slightly higher than that of cement mortar. (2) With the increase of PFRCC panel thickness, the bending failure load increases gradually, but the bending strength remains unchanged, and the peak displacement decreases gradually. The bending strength of the PFRCC panel is about 13.5 MPa, which is 2.7 times that of the matrix cement mortar. The above research provides a theoretical basis for the application of PFRCC panels in practical engineering such as permanent formwork of foundation beams and light-gauge steel stud concrete composite external wall panels.