Sylvester equation is often applied to various fields such as mathematics and control systems due to its importance. Zeroing neural network (ZNN), as a systematic design method for time-variant problems, has been proved to be effective on solving Sylvester equation in the ideal conditions. In this work, in order to realize the predefined-time convergence of the ZNN model and modify its robustness, two new noise-tolerant zeroing neural networks (NNTZNNs) are established by devising two novelly constructed nonlinear activation functions (AFs) to find the accurate solution of time-variant Sylvester equation in the presence of various noises. Unlike the original ZNN models activated by known AFs, the proposed two NNTZNN models are activated by two novel AFs, therefore possessing the excellent predefined-time convergence and strong robustness even in the presence of various noises. Besides, the detailed theoretical analyses of the predefinedtime convergence and robustness ability for the NNTZNN models are given by considering different kinds of noises. Simulation comparative results further verify the excellent performance of the proposed NNTZNN models, when applied to online solution of time-variant Sylvester equation. Index Terms-Zeroing neural network (ZNN), finite-time convergence, nonlinear activation function, Sylvester equation, timevariant problems. I. INTRODUCTION S OLVING Sylvester equation is often found in mathematics and control theory and applied to solve various important problems such as eigenvalue assignment [1], and image processing [2]. Therefore, it is a crucial issue to solve Sylvester equation by designing various different schemes. Numerical methods were usually used to solve the static Sylvester equation in the past [3]-[14], such as Bartels-Stewart, and Hessenberg-Schur iteration methods [8]-[14].