Static experiment simulated LT-MED seawater desalination condition was performed to measure antiscale efficiency of polyacrylic acid with a series of molecular weight. Scale inhibition ratio rose along with PAA dosage increased from 0.25 to 5 mg/L. When PAA dosage exceeded 2.25 mg/L, scale inhibition ratios above 95% were obtained. Interestingly, under the same dosage < 1.5 mg/L, the scale inhibition ratio was relatively low for PAA samples with larger M-n. All inhibition ratio curves exhibit a turning point when PAA dosage increases: for PAA with lower molecular weight, the turning point comes earlier. Data of inhibition efficiency shows correlation with PAA molarity rather than mass concentration, in other words, number of molecules behave more significance in scale inhibition process than chain length or number of functional groups. Antiscale mechanism was then established through data analysis above, and how molecular weight of PAA influences its lattice distortion ability on CaCO3 scale in LT-MED desalination system was explicitly elucidated. MD simulation was then carried out to demonstrate the proposed theory, by virtue of a new model concerning radius of critical nucleus.