Current specimen designs for biaxial tension-tension fatigue tests of composites are immature and often generate unexpected failure in fatigue tests, due to the lack of a design standard and the complexity of composite materials and multiaxial loads. So, designing composite structures with uniaxial testing is not sufficient due to the multiaxiality of the stress tensor. In this case, this paper aims to present an optimized specimen for multidirectional carbon fiber reinforced composites in biaxial tension-tension fatigue tests that fits with infrared thermography monitoring and provides a feasible specimen design by FE (Finite Element) simulation. Firstly, a design criterion is proposed to ensure a failure in the gauge region, a practical manufacturing approach, and optimal conditions for temperature measurement. An initial simulation model is established to find out the proper basic geometry followed by a more comprehensive simulation used for determining the dimension of the specimen. Consequently, the optimized specimen is designated as a cruciform shape with a reduced gauge region. Last, two stacking sequences of specimens, referred to as cross-ply [(0/90)6]s and quasi-isotropic [(0/45/90/-45)3]s, are tested in a simulation model that takes into account biaxial static and fatigue loadings, and then a few fatigue tests will be performed to validate the geometry. The favorable simulation and experimental results indicate the optimized specimen and design approach is well-suited for composite material in biaxial fatigue tests with temperature monitoring.