  We present a systematical study on the roles of electron correlation and
spin-orbit coupling in the isovalent Pd-doped superconductor SrPt$_3$P. By
using solid state reaction method, we fabricated the strong spin-orbit coupling
superconductors Sr(Pt$_{1-x}$Pd$_x$)$_3$P with strong electron correlated Pd
dopant of the $4d$ orbital. As increasing the isovalent Pd concentrations
without introducing any extra electron/hole carriers, the superconducting
transition temperature $T_c$ decreases monotonously, which suggests the
existence of competition between spin-orbit coupling and electron correlation
in the superconducting state. In addition, the electronic band structure
calculations demonstrate that the strength of electron susceptibility is
suppressed gradually by the Pd dopant suggesting the incompatible relation
between spin-orbit coupling and electron correlation, which is also consistent
with experimental measurements. Our results provide significant insights in the
natures of the interplay between the spin-orbit coupling and the electron
correlation in superconductivity, and may pave a way for understanding the
mechanism of superconductivity in this 5d-metal-based compound.
