The mutant promoter of human telomerase reverse transcriptase (hTERT) shows high transcriptional activity in bladder cancer cells. Some up-regulated microRNAs (miRNAs) are reported as oncogenic factors in bladder cancer. Previous studies report that miRNAs can be inhibited by base-pairing interactions. The purpose of this study is to construct a synthetic device driven by mutant hTERT promoter to suppress four up-regulated miRNAs and to verify its effects on phenotypes of bladder cancer cells and human normal cells. Tandem bulged miRNA binding sites targeting oncogenic miRNAs were inserted into the 3' untranslated region (3' UTR) of mutant hTERT promoter-driven Renilla luciferase gene to construct a synthetic tumor-specific device, miRNA sponges. A negative control was generated by using tandem repeated sequences without targeting any known miRNA. Bladder cancer cells (T24, 5637, UM-UC-3) and human fiber cells (HFC) were transfected with devices. Various functional assays were used to detect the effects of this device. The activity of the mutant hTERT promoter detected by luciferase assay was about three times as large as the wild-type hTERT promoter in bladder cancer cells, while it could not be measured in HFC. Other assays indicated that the synthetic device can significantly inhibit cell growth, decrease motility, and induce apoptosis in bladder cancer cells but not in HFC. A synthetic biology platform is employed to construct tumor-specific miRNA sponges that can be used to target oncogenic miRNAs to inhibit the progression of bladder cancer cells without affecting normal cells.