Abstract: For controlling mid-spatial error in the magnetorheological finishing，experiments were carried out to analyze the relationship between the dwell time and mid-spatial error. Based on the matrix method of dwell time algorithm，the fluctuation of the time matrix along the feed direction of polishing reflects the discontinuity of velocity，which will cause the mid-spatial error. This paper advanced to filter the dwell time and smooth the velocity，which can make the velocity of the adjacent points more close，so the polishing wheel only needs a small acceleration and very little time to complete the entire acceleration，so as to reduce the error caused by the fluctuation of speed. Through computer simulation and experiments，it verificated that to give the dwell time a small disturbance，the PSD curve will diverge， and the dwell time filtered will inhabit the mid-spatial error to some extent.