Abstract: The needle penetration test was used to comparatively analyze the needle penetration strength, uniaxial compressive strength, and their correlation for intact mudstone and remolded mudstone, the variation patterns of the mechanical properties of mudstone under different moisture contents was also explored. In addition, a drip-water treatment during penetration is introduced to simulate water infiltration and to investigate its effects on penetration resistance and deformation characteristics. The results indicate that intact rock exhibits pronounced data scatter, particularly within the low penetration-strength range where the responses fluctuate substantially, whereas remolded mudstone demonstrates markedly improved repeatability. Moisture content exerts a significant influence on compressive strength: specimens in the oven-dried condition show substantially higher compressive strength and penetration resistance than those at 3%, 5%, and 7% water contents, which consistent with trends reported by other testing methods. Water treatment induces an inflection and subsequent decline in the load–penetration depth curve, indicating rapid softening and a significant deterioration of strength and penetration performance; under load-bearing conditions, drip-water infiltration leads to pronounced reductions, with the compressive strength in the dried state decreasing by as much as 58.7%. The results provide further experimental evidence for understanding moisture-induced weakening in mudstone and confirm the feasibility of needle penetration testing for the rapid and reliable evaluation of the mechanical properties of soft rocks.