Gastrointestinal (GI) tract related diseases are common and deadly. To avoid the diseases developing into an advanced stage, early inspection and intervention are crucial. However, existing inspection methods are either uncomfortable or lack of active steering. Therefore, it is necessary to investigate a locomotion method that is friendly to the GI tract. This paper proposed a novel soft snail robot targeted for active locomotion inside the GI tract. By studying the locomotion mechanism of gastropods, a snail robot propelled with longitudinal travelling wave was designed. The wave was generated by deflating and inflating a series of chambers. The snail robot could adhere to different surfaces, including the ex vivo porcine intestine, by the mucus underneath. Experiments were designed to study the performance of the proposed snail robot. Results show that, the soft snail robot could crawl over different substrates including hard substrate, soft substrate and biological tissue. The speed of the snail is affected by multiple factors, including the surface condition, wave frequency and the mucus concentration.