Optical tweezers (OTs) are an important tool for the viscosity measurements in microrheology, and passive techniques have the features of being simple and need no external force generations. Current passive methods using OT always first calibrate the potential stiffness and then do parameter fittings to obtain the viscosity. Here, we introduced and demonstrated a passive viscosity estimation method for low-viscous microfluids using OTs without stiffness calibration and parameter fitting. By Brownian trajectory tracking of single trapped bead, the viscosity coefficients of water and NaCl solutions are quickly obtained with small deviations (typical <10  %  ) from the reference values. Besides, we introduce estimations for the commonly used voltage-to-displacement conversion factor, and the consistency check between the estimations and calibrations is used to represent the estimation quality. The whole process is very convenient for automatic processing. Further matrix operations are proposed and tested, which are expected to be integrated with holographic OTs and optical fiber traps for distributed multidimensional measurement.