Abstract: Top-up operation mode of Shanghai Synchrotron Radiation Facility (SSRF) needs the linac to supply a stable, continuous and high quality beam. The principle of an 8-pole strip-line-type beam position monitor (BPM) and measurement of the beam energy spread at low energy transform line from the linac to the booster are described, and a proportion and integration (PI) control algorithm is used to complete the feedback control of beam energy spread, which uses RF (radio frequency) amplitude or phase as the adjusting parameter. A long term stability of the beam energy is realized, which ensures the high rate of the beam injection of the linac.