  We report $^{195}$Pt nuclear magnetic resonance (NMR) measurements on
topological superconductor candidate YPtBi which has the broken inversion
symmetry and topological non-trivial band structures due to the strong
spin-orbit coupling(SOC). In the normal state, we find that Knight shift $K$ is
field- and temperature-independent, suggesting that the contribution from the
topological bands is very small at low temperatures. However, the spin-lattice
relaxation rate 1/$T_1$ divided by temperature ($T$), 1/$T_1T$, increases with
decreasing $T$, implying the existence of antiferromagnetic spin fluctuations.
In the superconducting state, no Hebel-Slichter coherence peak is seen below
$T_{\rm c}$ and 1/$T_1$ follows $T^{3}$ variation, indicating the
unconventional superconductivity. The finite spin susceptibility at
zero-temperature limit and the anomalous increase of the NMR line width below
$T_{\rm c}$ point to a mixed state of spin-singlet and spin-triplet(or
spin-septet) pairing.
