  Multiple mechanisms for extremely large magnetoresistance (XMR) found in many
topologically nontrivial/trivial semimetals have been theoretically proposed,
but experimentally it is unclear which mechanism is responsible in a particular
sample. In this article, by the combination of band structure calculations,
numerical simulations of magnetoresistance (MR), Hall resistivity and de
Haas-van Alphen (dHvA) oscillation measurements, we studied the MR anisotropy
of SiP$_{2}$ which is verified to be a topologically trivial, incomplete
compensation semimetal. It was found that as magnetic field, $H$, is applied
along the $a$ axis, the MR exhibits an unsaturated nearly linear $H$
dependence, which was argued to arise from incomplete carriers compensation.
For the $H$ $\parallel$ [101] orientation, an unsaturated nearly quadratic $H$
dependence of MR up to 5.88 $\times$ 10$^{4}$$\%$ (at 1.8 K, 31.2 T) and
field-induced up-turn behavior in resistivity were observed, which was
suggested due to the existence of hole open orbits extending along the $k_{x}$
direction. Good agreement of the experimental results with the simulations
based on the calculated Fermi surface (FS) indicates that the topology of FS
plays an important role in its MR.
