We use OQPSK because of envelope of signal but my question is
"In Phase Shift Keying(or QPSK) the amplitude of the transmitted signal is constant then how the envelope will change in OQPSK"??
Note that a QPSK signal using a band-limited pulse has an envelope that passes through zero every time there is a phase transition of $\pi$. So its envelope is not constant if all symbol transitions are allowed.
Offset QPSK (OQPSK) doesn't have phase transitions of $\pi$. By staggering the $I$ and $Q$ signals by half a symbol interval, the maximum phase transition is $\pi/2$, leading to an approximately constant envelope. Slight droops in the envelope (occurring at phase transitions of $\pi/2$) can be eliminated by hard-limiting.
Note that with an appropriate choice of the transmit pulse, phase transitions in OQPSK can be completely avoided. This results in a modulation scheme with an exactly constant envelope. One example of such a continuous-phase modulation is minimum-shift keying (MSK).
For more information on OQPSK take a look at this question and its answers. Also browse this site for other questions on OQPSK.