In this paper, we study the qualitative properties of single blow-up solutions to the nonlocal equations with slightly subcritical exponents \(\begin{aligned} -\Delta u=(|x|^{-(n-2)}*u^{p-\varepsilon })u^{p-1-\varepsilon }\quad \text{ in }~~\Omega ,~~ u=0\quad \text{ on }~~\partial \Omega , \end{aligned}\) where \(\Omega \) is a smooth bounded domain in \(\mathbb {R}^n\) for \(n=3,4,5\) , \(*\) denotes the standard convolution, \(\varepsilon >0\) is a small parameter and \(p=\frac{n+2}{n-2}\) is \(\mathcal {D}^{1,2}\) energy-critical exponent. By exploiting various local Pohozaev identities and blow-up analysis, we provide a number of estimates on the first \((n+2)\) -eigenvalues and their corresponding eigenfunctions, and examine the qualitative behavior of the eigenpairs \((\lambda _{i,\varepsilon }, v_{i,\varepsilon })\) to the linearized problem of the above nonlocal equations for \(i=1,\cdots ,n+2\) . As a corollary, we derive the Morse index of a single-bubble solution in a nondegenerate setting.