Optical thermography is an important non-destructive testing (NDT) method which has been widely used in the fields of modern aerospace, renewable energy, nuclear industry, etc. The excitation source is a crucial device for the optical thermography system whose performance has a decisive effect on the detection results. Previous thermography NDT studies mainly focused on the physical mechanism, applications and signal processing algorithms. However, the design of the excitation source is rarely discussed. Due to the wide frequency range as well as the high power excitation requirements, it is a challenging task to develop a multi-mode excitation source for thermography NDT. This paper presents a novel design of the excitation source with a structure topology that combines the circuit with low frequency sinusoidal generation and a chopper circuit. It intimately satisfies the requirements of multiple-mode excitation for optical thermography. These include pulsed thermography (PT), lock-in thermography (LT), step heating thermography (ST), pulsed phase thermography (PPT), frequency modulated thermal wave imaging (FMTWI) and barker coded thermal wave imaging (BCTWI). The proposed topology, operating principle and the design procedure of the circuit have been investigated in details. A 2 kW prototype with a frequency range of 0.01 Hz–100 kHz has also been implemented. Validation of the proposed method has been undertaken to detect inner defects of both on a composite sample and a lead-steel sample with bonded structure.