Electron pairing along with phase coherence generates superconductivity below the critical temperature (T_c). In underdoped high-T_c cuprates, these two quantum phenomena may occur at separate temperatures, which was lately confirmed in the quasi-two-dimensional (quasi-2D) iron chalcogenide superconductors. Here, we report a systematic investigation on the pre-pairing behavior in a triclinic iron pnictide superconductor (Ca_0.85La_0.15)₁₀(Pt₃As₈)(Fe₂As₂)₅ with T_c ≈ 30 K, where the superconductivity is quasi-2D manifested by the Berezinskii-Kosterlitz-Thouless behaviors. Inelastic neutron scattering experiments unambiguously reveal a spin resonance peak around E_R = 13 meV in the superconducting state, but its intensity continuously decreases when warming up across T_c, accompanied with an anomaly around T* ≈ 45 K in spin correlations, and a suppression by an in-plane magnetic field persisting to the same temperature. Below T*, a significant Nernst signal and a reduction of density of states at the Fermi level are also observed. These results suggest that the precursor of spin resonance is highly related to the preformed Cooper pairs driven by phase fluctuations, much like the pseudogap case in cuprates.