Most research works in transcoding proxies in mobile computing environments are on the basis of the traditional client-server architecture and do not employ the data broadcast technique. In addition, the issues of QoS provision and energy conservation are also not addressed in the prior studies. In view of this, we design in this paper a QoS-aware and energy-conserving transcoding proxy by utilizing the on-demand broadcasting technique. We first propose a QoS-aware and energy-conserving transcoding proxy architecture, abbreviated as QETP, and model it as a queueing network consisting of three queues. By analyzing the queueing network, three lemmas are derived to estimate the load these queues. We then propose a version decision policy and a service admission control scheme to provide QoS in QETP. The derived lemmas are used to guide the execution of the proposed version decision policy and service admission control scheme to achieve the given QoS requirement. In addition, we also propose a data indexing method to reduce power consumption of clients. To measure the performance of the proposed architecture, three experiments are conducted. Experimental results show that the average access time reduction of the proposed scheme over traditional client-server architecture ranges from 45% to 75%. Experimental results also show that the proposed scheme is more scalable than traditional client-server architecture and is able to effectively control the system load to attain the given QoS requirements. In addition, the proposed scheme is able to greatly reduce average tuning time of clients at the cost of a slight increase (around 5% in our experiments) in average access time.