The phase difference between displacement and acceleration of a particle in a simple harmonic motion is:

Displacement $\left(x\right)$ equation of SHM

$x=A\sin (\omega t+\varphi )$                   ....(1)

$\frac{dx}{dt}=A\omega \cos \left(\omega t+\varphi \right)$

acceleration $\left(a\right)=\frac{d^{2}x}{d{t}^2}$

$a=-{\omega }^{2}A\sin \left(\omega t+\varphi \right)$

$a={\omega }^{2}A\sin \left(\omega t+\varphi +\pi \right)$         ....(2)

from (1) & (2), phase difference between displacement and acceleration is $\pi .$