Sample preparation is one of the essential processes for most biochemical assays on biochips. Many studies have been conducted for dealing with the reactant minimization problem during sample preparation. Nevertheless, those approaches try to minimize reactant consumption at the cost of more extra operations, which may lead to deterioration of reactant and even wrong results. In this paper, we propose a time-constrained sample preparation algorithm for reactant minimization on digital microfluidic biochips (DMFBs). Starting from a given reactant-minimized solution, the proposed algorithm tries to incrementally reduce the operation count at the cost of more reactant consumption to meet the given timing constraint. The experimental results show that our algorithm achieves a 33% reactant reduction over a delay-optimal method with the same operation time. Meanwhile, the proposed method can save 5% operation count as compared with a state-of-the-art reactant minimization algorithm under the same reactant consumption.