Real-time measurement of the transient optical signal provides invaluable information about the dynamic processes, such as optical data stream monitoring in all-optical network, velocity interferometry and performance evaluation of single optical pulse in physics of intense fields. An all optical system based on wide spectral pump four wave mixing optical-optical sampling and time stretch is reported to measure the transient optical signal in real time. The proposed technique samples the optical signal with super-resolution, converts the single wavelength signal to wide spectral signal and stretches it using dispersive Fourier transformation. The process of four wave mixing and time stretch in theory are analyzed and its feasibility is verified.