In this work, we demonstrate a novel cascaded fiber-optic structure depending on Fabry-Perot interference and anti-resonance (AR) effect for seawater salinity and temperature simultaneous measurement. An in-fiber microchannel Fabry-Perot interferometer (FPI) is obtained in a single-mode fiber (SMF) by using femtosecond laser radiation and hydrofluoric acid corrosion, which is filled with thermosensitive polymer and able to measure the temperature. A section of hollow-core fiber (HCF) is fused between the SMFs with the square micro-slots ablated on the ends, and the liquid can easily flow into the HCF. To our knowledge, this is the first time that the capillary-type HCF was used as a microfluidic device to measure seawater salinity. Through model analysis and experimental tests, it is confirmed that the proposed sensor can simultaneously measure the salinity and temperature, the corresponding minimum detectable resolutions reach 0.0008‰ and 0.001 ℃, respectively. The performance tests show that the sensor has accurate measurement results, good stability, and repeatability. Besides, the proposed HCF-based internal liquid analysis structure is a promising approach for highly reliable and ultrasensitive biochemical sensing.