Air pollution poses a serious threat to public health, especially in developing countries, and how to reduce it has gained great attention over the past decades. In this work, a hierarchical-structured composite, ultrafine TiO2 encapsulated in a nitrogen-doped porous carbon framework, was fabricated and used as a photocatalyst to degrade ammonia gas. In addition, a gas sensor test system lined with a photocatalytic apparatus was proposed to estimate the degradation of ammonia gas, which allowed for facile in situ analysis and assessment of the photodegradation process of the toxic gas. Due to the large surface area, abundant pore structure, good gas adsorption property, extended light harvesting capacity and charge transfer capability of the nitrogen-doped porous carbon, the as-prepared ultrafine TiO2 encapsulated in the nitrogen-doped porous carbon framework displayed excellent photocatalytic activity and degraded ammonia gas with 100% efficiency after only 5 min of light irradiation.