Hydrophilic interaction LC for the separation of four antihypertensive drugs was optimized by both univariate and multivariate methods. The column efficiency, resolution, and separation time were used as the three assessment parameters. The best separation condition of 97% ACN with 3% aqueous buffer containing 50 mM ammonium acetate at a pH of 3.0 was obtained by the two optimization methods. The multivariate optimization, orthogonal array design herein, was demonstrated to be a little tedious, but afforded a much better understanding of underlying separation factors. The content of ACN in the mobile phase contributed most significantly to separation. Furthermore, sample diluent and injection volume were found to influence the chromatographic performance. To match the hydrophilic interaction LC mobile phase, a proper sample pretreatment method, salting-out liquid–liquid extraction, in which ACN was the extractant, was chosen. Since reserpine was unstable under both acidic and alkaline conditions, it was not studied in this part. The optimal salting-out liquid–liquid extraction parameters were as follows: 400 μL ACN was added to 1 mL sample solution containing 500 mg NH4Cl at a pH of 14.0. The linearity ranged from 0.01 to 1.00 μg/mL with r2 > 0.9937. The LODs were between 1.9 and 2.5 ng/mL. The developed method was applied to the environmental water sample with good performance.