The invention discloses a high-temperature portable long-optical-distance ammonia gas ultraviolet spectrum detection method. The method is characterized by comprising the following steps: (S1) carrying out sampling from a sample source land; (S2) configuring an ultraviolet spectrum detection device; (S3) introducing an ammonia gas sample into a gas absorbing tank, starting the ultraviolet spectrumdetection device to drive an ultraviolet light source to generate ultraviolet light, wherein the ultraviolet enters the gas absorbing tank through a collimator, is reflected in an ultraviolet light reflection mirror set, is emitted out through the other side of the gas absorbing tank and is received by a spectrograph; and (S4) analyzing and processing data acquired by the spectrograph by virtue of a computer. The method has the beneficial effects that after ammonia gas is introduced into the gas absorbing tank, and the ultraviolet light irradiates into the gas absorbing tank, is repeatedly reflected by virtue of the reflection mirror set, is emitted out of the gas absorbing tank and is then detected by virtue of the spectrograph, so that a detection result is high in detection precision and has a good detection effect.