Background: Shanghai Soft X-ray FEL is under design and construction at Shanghai Institute of Applied Physics（SINAP）.Purpose: The stability of the existing SSRF Linac modulator has achieved 500ppm.But the Soft X-FEL requires 100PPM.Methods: This paper describes control strategy based on pulse amplitude feedback technology

and gives the design of signal conditioning circuit in more details.The signal conditioning circuit contains amplitude fading

LPF and cutting etc.Results: The relevant experiments show that the circuit reaches required function and stability.Conclusions: Feedback control strategy is useful for the existing modulator.