To measure the ultra low current in a quadrupole mass spectrometer(QMS) by a preamplifier,low noise and wide bandwidth is the key to this system for its current is from 10^-11 A to 10^-6 A and varies quickly.So a Non-inverting Input Composite Preamplifier(NIICP) is designed,and the effect of the open-loop gain on optimizings the frequency responses of signal and noise is analyzed.NIICP can optimize the openloop gain to reduce intrinsic noise in the lower magnification and to recover high frequency signal damped down by the low pass transfer function of large feedback resistance and parasitic capacitance in the higher magnification.Compared with Inverting Input Composite Preamplifier(IICP) in the commercial QMS,the NIICP can reduce noise by 5%～30% in the 10⁶～10⁷ V/A magnification,and can expand signal bandwidth by 1.8 times and increase MS peak amplitude by 10%～15% in the 10⁸ V/A magnification.Experimental results show that the wide-band low-noise NIICP improves the signal-to-noise radio of MS peak in large dynamic range of 10⁶～10⁸ V/A.