Let Λ(n) be the von Mangoldt function. Let \({1\!\!1}_{\mathbb{P}}(n)\) be the characteristic function of prime numbers and let [t] be the integral part of real number t. In this paper, we prove that the asymptotic formula \(\mathop \sum \limits_{n \le x} \Lambda ([{x \over n}]) = \left(\mathop \sum \limits_{d = 1}^\infty {{\Lambda (d)} \over {d(d + 1)}}\right)x + {O_\varepsilon }({x^{7/15 + \varepsilon }})\) holds as x → ∞, where ε > 0 is an arbitrarily small positive number and c > 0 is a positive constant. This improves some recent results of Zhang [J. Number Theory, 2024, 257: 163–185] and of Lü [Colloq. Math., 2024, 177(1–2): 11–19], which require \({7 \over {15}} + {1 \over {195}}\) and \({{22} \over {47}}\) in place of \({7 \over {15}}\) , respectively. We also improve some results of Ma–Chen–Wu [Int. J. Number Theory, 2019, 15(3): 597–611] and of Zhou–Feng [Rocky Mountain J. Math., 2024, 54(2): 623–629].