By utilizing the feature that adding Si is able to effectively increase Ac1 and Ac3 temperatures of ferritic-martensitic (FM) steels, four FM steel sheets with different Si additions (0–1.0 wt%) were prepared by hot rolling at the same intercritical temperature to obtain various heterostructures. Their mechanical properties were evaluated and well correlated with detailed microstructural characteristics revealed by multiple characterization methods. Results show that the 0 Si specimen has a nearly complete martensitic structure; the 0.4 Si specimen consists of ∼23.5 vol% island-like ferrites embedded in martensites exhibiting a typical bi-modal structure (BMS); the 0.7 Si specimen consists of ∼46.8 vol% lamellar ferrites alternating with lamellar martensitic structures corresponding to a typical heterogeneous lamella structure (HLS); the 1.0 Si specimen is mostly comprised of ferrites no longer belonging to typical heterostructures. With increasing Si content, sizes of martensitic laths gradually decrease while the ferritic grains are coarsened. The 0.4 Si and 0.7 Si specimens exhibit high values (∼17 GPa%) of the product of ultimate tensile strength (UTS) and total elongation (TE), demonstrating a good strength-ductility synergy. Comprehensive analyses reveal that the 0.4 Si specimen with BMS exhibits a considerable hetero-deformation induced (HDI) strengthening effect, facilitating the production of the highest UTS (1476.3 MPa), while the 0.7 Si specimen with HLS shows a stronger HDI strain hardening effect leading to a higher TE (12.5%). This study could offer an important reference for designing and processing of FM steels with excellent strength-ductility synergy.