  In this paper, we perturb the global center of the planar polynomial vector
fields $\mathcal{X}(x,y)=(-y(x^2+a^2),x(x^2+a^2))$ ($a\neq0$) inside cubic
piecewise smooth polynomials with switching line $y=0$. By using average
function of first order, we prove that the sharper bound of the number of limit
cycles bifurcating from the period annulus is 6.
