The influence of the Amplitude Shift Keying(ASK) modulation depth on energy transmission and data transmission between a reader and a tag in the passive Radio Frequency Identification(RFID) system was investigated. Expressions of energy transmission efficiency and data transmission reliability varying with the modulation depth were deduced, and their performance curves were given. These expressions and curves are helpful to the selection of ASK modulation depth for an RFID system and the power design of a reader, and they can be used to guide application design and development of the passive RFID systems in practice.