This article present a design method- a triple coating surface waveguide for augmented reality (AR). Recently, with its blooming developments over decades, AR has established distinguishing features that makes it widely applicable among various fields. Overlapping virtual images onto the external view, AR has contributed to applications, such as education, medical surgery, engineering, entertainment, image-guided navigation and even military. However, there are still two major challenges for AR: (1) The inconvenience brought by the bulkiness of AR head mounted display (HMD) devices; (2) The limited magnitude of the field of view (FOV). To improve the drawbacks above, we propose a triple coating surface waveguide that is thinner than the traditional designs of the AR HMD devices, We separate our waveguide into two parts: (1) The free-form surface collimator based on the n0n-pupil forming system, constructed by four high order term extended polynomial surfaces, (2) The triple coating surface directly attached at the end of the waveguide. Traditionally, the FOV relies on the size of the single couple out surface as the single surface determines all wave reflections. Our triple coating surface allows more flexibility on the reflections of optical rays. Each coating possess a critical angle that filters and reflects the optical rays that have a matching incident angle. In other words, the optical rays with different incident angles are reflected at corresponding coating surfaces. We have achieved a large FOV of 40 degree with a 3 mm thick waveguide.