A new ultrafast interrogation is proposed based on the peanut shaped tapered fiber (PSTF) interference structure and time-stretching method. The single mode fiber (SMF) is tapered by tapering machine to achieve a minimum core diameter width of 10.86 μm. Thanks to the peanut structure as a beam expander and collector, multiple high order modes have been excited. The interference between core mode and cladding modes has a high response to changes in surrounding salinity, with a salinity sensitivity of 0.302 nm/‰ and a corresponding refractive index (RI) sensitivity of up to 1800.535 nm/RIU. The time-stretching method is put forward and experimentally demonstrated. Once the femtosecond light pulse is modulated by the PSTF interferometer, the pulse light carrying information is transmitted to a dispersion compensation fiber (DCF) to achieve wavelength to time mapping and the spectrum in the time domain can be directly collected on an oscilloscope (OSC). By monitoring the time shift of salinity modifications, a salinity sensitivity of -0.128 ns/‰ and a real-time acquisition speed of 50 MHz for the sensing system is realized. The proposed salinity monitoring system has the advantages of high sensitivity, fast response speed, low cost and high accuracy. It has significant potential for real-time monitoring and timely provision of salinity data, which can help improve water quality and achieve scientific aquaculture. The proposed interrogation technology has broad application prospects for any real-time monitoring based on fiber optic interference modulation.