The real-time orthogonal frequency division multiplexing (OFDM) signal is generated based on Xilinx field programmable gate array (FPGA),which includes p seudo-random binary sequence (PRBS) ge nerator,inverse fast Fourier transform (IFFT) with completely parallel c omputing technology,and the design of digital-to-analog conversion (DAC) inter face circuit.The constellation of demodulated OFDM signal has a good convergence ,and the corr esponding error vector magnitude (EVM) is 4.42%.Bit error rate is significantly in creased due to the different rotation phases for sub-carriers,when the sampling frequency offset exists between transmitter and receiver,and we get improve d system performance after sampling frequency synchronization.The direct-detect ion optical OFDM experimental system is established,and then the off-line digital s ignal pro cessing after 200km standard single mode fiber transmission is taken.An optica l power penalty of less than 0.6dB is shown,compared with back-to-back (BTB) o peration.The experimental results show that the designed real-time optical OFD M transmitter has good performance.