Changling gas field is a large scale volcanic gas field in Jilin oil field. Fractures exist widespreadly in Yingcheng Formation reservoir. During the developmental procedure of the gas field, the bottom water of the gas reservoir invaded into reservoir along the fractured system. The water occupied the original gas storage space gradually. Based on the assessment and observation of volcanic reservoir fractures, the fractures are classified according to the genetic type, occurrence characteristics and fractured width. The detailed characteristics of volcanic reservoir is summarized after study of FMI logging graphs and section observation. In order to describe the phenomenon of water invasion in volcanic body, the CS103 volcanic body is taken as an example to be evalued. The volume of water intrusion is calculated with the condition that various data is acquired during past exploitation years. It is confirmed that the complicated fracture system in volcanic rocks is conducible to gas flow from the reservoir pores to the bottom of gas wells. At the same time, the bottom water intruded into the volcanic reservoir along fractures as well. This paper provides beneficial reference for the research or evaluation on fractures in similar volcanic gas reservoir.