A new fourdimensional autonomous chaotic system was presented.First,some basic dynamical properties of this system with integer order were studied.The dynamic characteristics of the fractionalorder system varying with the differential operator orders with fixed parameter are analyzed by numerical simulation and the power spectrum .The results show that, with the variation of system parameter at a differential operator order of 0.85,the fractionalorder new system enters into chaotic state by transient chaos and boundary crisis bifurcation.Based on Chebyshev orthogonal neural network,a stablility theorem［14］and the fractionalorder PI switching surface,a novel adaptive nonlinear observer with a compensator was designed for a class of fractionalorder chaotic system, where the nonlinear portion of the structure cannt be evaluated.The projective synchronization of fractional order new chaotic system can be achieved.Numerical simulation certifies the effectiveness of the method.