As the rapid increase of DNA sequences, there is a crucial need of effective methods to detect genes and genes’ structures, among which splice sites prediction plays a key role. There are conservative segments on the junctions between introns and exons, which can help us predict splice sites by computational methods; however it is unclear that which nucleotides contribute to the splicing process, so it is necessary to select a suitable set of features to accomplish the prediction of splice sites. A length-variable Markov model is proposed in this paper. By the length-variable model, a suitable subset of features can be chosen as the detecting features for each candidate splice site according to the ratio of likelihood at each position. The results of our experiments show that our models not only achieve higher prediction accuracy than the basic Markov model and some present methods, but also preserve the feature of low time cost as the basic Markov model does.