The effect of the acetylenic-modified polysiloxane emulsion defoaming agent on the surface tension, density, viscosity, pH value and writing performance of the gel ink was investigated. The results showed that the surface tension of the gel ink decreased slightly and the viscosity and pH value of the gel ink were not affected by adding the defoaming agent. The optimizational dosage of defoaming agent was 0.042wt%~0.043wt%. The pens filled with such gel ink behave good writing performance. And the uniformity of the amount of the gel ink consumed during writing was improved.