  While recent advances in band theory and sample growth have expanded the
series of extremely large magnetoresistance (XMR) semimetals in transition
metal dipnictides $TmPn_2$ ($Tm$ = Ta, Nb; $Pn$ = P, As, Sb), the experimental
study on their electronic structure and the origin of XMR is still absent.
Here, using angle-resolved photoemission spectroscopy combined with
first-principles calculations and magnetotransport measurements, we performed a
comprehensive investigation on MoAs$_2$, which is isostructural to the $TmPn_2$
family and also exhibits quadratic XMR. We resolve a clear band structure well
agreeing with the predictions. Intriguingly, the unambiguously observed Fermi
surfaces (FSs) are dominated by an open-orbit topology extending along both the
[100] and [001] directions in the three-dimensional Brillouin zone. We further
reveal the trivial topological nature of MoAs$_2$ by bulk parity analysis.
Based on these results, we examine the proposed XMR mechanisms in other
semimetals, and conclusively ascribe the origin of quadratic XMR in MoAs$_2$ to
the carriers motion on the FSs with dominant open-orbit topology, innovating in
the understanding of quadratic XMR in semimetals.
