This paper reports the feasibility of using Riemann-Hilbert method (RHM) to reveal the soliton dynamics with variable velocity and amplitude in mixed spectral nonlinear integrable systems. To achieve this goal, we consider a novel mixed spectral complex mKdV equation (mscmKdV) with time-varying coefficients. Firstly, Lax representation of the timevarying coefficient mscmKdV equation is first abstracted from Ablowitz-Kaup-Newell-Segur (AKNS) matrix spectral problem equipped with a mixed spectrum. With the help of the Lax representatoin, a Riemann-Hilbert problem (RHP) associating with the mscmKdV equation is then established. Due to the confirmation of solvability of the RHP, scattering data used for recostructing the potential are further determined, and N-soliton solution of the mscmKdV equation is finally obtained. In addition, the one-and two-soliton solutions are shown to explore the characteristics of soliton dynamics in the mscmKdV equation. It is revealed fromthe mscmKdV equation that compared with the constant velocity and amplitude propagation of a soliton in the background of constant-coefficient isospectral model, the one of variablecoefficient isospectral system propagates with variable velocity but constant amplitude, while for the case of non-spectrum, whether its coefficients are variable or not, soliton always propagates with variable velocity and amplitude.