In a traveling wave tube, efficiency of multistage depressed collector (MDC) is very important because it is closely related to total efficiency. The correct estimate of the efficiencies of MDC and TWT can help us predict TWT's whole function and provide a theoretical guidance for developing pertinent software, which therefore plays an important role in optimizing MDC and improving TWT total efficiency. Although formula for predicting MDC efficiency was given by Kosmahl in 1980, whose estimation is much higher than the measured value, a more accurate formula is still necessary. Firstly, the concept of dissipated common difference is used in this paper to estimate the efficiencies of MDC and TWT and then new estimate formulas are obtained by making a model of arithmetic triangular energy distribution for spent beam. It is expected that new formulas give predictions closer to the measured values than the Kosmahl's evaluation. Finally, expression for optimal MDC electrodes is given on the basis of two extreme values, i.e., maximal MDC efficiency and minimal total dissipated energies on all electrodes. The prediction from the expression is reasonable and accurate.