This paper presents the SyntaxTree Aligner, a web-based language-independent syntactic trees construction and alignment platform for monolingual and bilingual sentences. This platform has served as an assisting tool for language annotator to the construction of high-quality parallel treebank, in a visual way. It combines the strengths of some existing treebank construction tools, i.e. well-defined Tiger-XML representation for syntactic structures, Stockholm-XML format of tree alignments, and monolingual (bilingual) tree annotation and revision interface, while addressing some of their common shortcomings, i.e. tools have to be installed in the local computer. The platform supports various annotation schemes, such as the annotation scheme used in Penn Treebank, and the universal tag set. We report the ongoing construction work of two parallel treebanks, Chinese-English and Chinese-Portuguese, using SyntaxTree Aligner. The SyntaxTree Aligner is freely available at http://nlp2ct.cis.umac.mo.