In this paper a Vernier effect based sensor is analyzed and demonstrated experimentally in a tri-microfiber coupler (Tri-MFC) and polarization-maintaining fiber (PMF) loop interferometer (Tri-MFC-PMF) to provide ultrasensitive refractive index and temperature sensing. The main novelty of this work is an analysis of parameters of the proposed Tri-MFC-PMF with the objective of determining the conditions leading to a strong Vernier effect. It has been identified by simulation that the Vernier effect is a primary factor in the design of Tri-MFC-PMF loop sensing structure for sensitivity enhancement. It is furthermore demonstrated experimentally that enhancing the visibility of the Vernier spectrum in the Tri-MFC-PMF allows to achieve an ultrahigh refractive index and temperature sensitivity with improved measurement accuracy. Specifically it is shown that small values of the total phase difference ( π / 16 + Nπ )∼( π / 4 + Nπ ), where N is an integer, accumulated over the PMF loop and Tri-MFC loop result in a strong Vernier effect. Experimentally an ultrahigh refractive index sensitivity of -20588 nm/RIU and temperature sensitivity of 0.019 nm/C are demonstrated by utilizing the stronger Vernier effect with clear Vernier spectrum. This analysis of the parameters may be useful to future researchers seeking to increase the measurement accuracy of sensors by enhancing the spectral visibility of the Vernier effect in other types of fiber optic interferometers.