We report the results from textit{AstroSat} observations of the transient Galactic black hole X-ray binary MAXI J1535-571 during its hard-intermediate state of the 2017 outburst. We systematically study the individual and joint spectra from two simultaneously observing textit{AstroSat} X-ray instruments, and probe and measure a number of parameter values of accretion disc, corona and reflection from the disc in the system using models with generally increasing complexities. Using our broadband ($1.3-70$ keV) X-ray spectrum, we clearly show that a soft X-ray instrument, which works below $sim 10-12$ keV, alone cannot correctly characterize the Comptonizing component from the corona, thus highlighting the importance of broadband spectral analysis. By fitting the reflection spectrum with the latest version of the textsc{relxill} family of relativistic reflection models, we constrain the black holes dimensionless spin parameter to be $0.67^{+0.16}_{-0.04}$. We also jointly use the reflection spectral component (textsc{relxill}) and a general relativistic thin disc component (texttt{Kerrbb}), and estimate the black holes mass and distance to be $10.39_{-0.62}^{+0.61} M_{odot}$ and $5.4_{-1.1}^{+1.8}$ kpc respectively.
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