In this paper, we have reviewed the investigations on the ion pick-up and two-stream instability in cometary tail, and then we go on the discussion. We adopted a reasonable particle distribution function ,used a simplified Van Kampen judgement to analyze the possibility of the ion pick-up caused by two-stream instability in the cometary tail. On calculation , we get an energy limitation (E1-E2) between the solar wind particles and the cometary particles. When the energy difference between the solar wind particles and the cometary particles is between E1 and E2, the pick-up of ions in the cometary tail is more effective.