The slope of the curve optical power VS. voltage at ATT=1 dB is required to be less than 1 dB/V, which is not met by most of VOA. The traditional method is to control the gain of multi-stage operational amplifiers by the output feedback. However, the discontinuity points on gain curve are inevitable. Based on the pure non-linear circuit, the requirement can be met. Besides flattening the slope @ATT=1 dB at specific points, the curve still keeps the original value and there is not any discontinuous points. The theory is analyzed, the circuit is simulated by Cadence Orcad Pspice software，and the conditions of zero slope the derivative of the curve is zero) are derived. The diversity of JFET and MATT VOA is analyzed and the temperature characteristics are investigated. Based on the technique above, the circuit occupies small space and its cost is low, 8 pcs MATT VOA modules are made successfully and the optical and electrical test results are shown.