Yeah！
I use the standard 7842 CVBS scripts to configure the part!
This problem does not appear when the input is HDMI! And I don't think that problem is triggered by the PC capture software since it will not be a problem when I use the another CVBS de-modulation chip to de-modulate the CVBS signal.
The configuration of 7842 is listed on the following:
writeADV7842(IO_I2C_ADDR,0x00,0x00);
writeADV7842(IO_I2C_ADDR,0x01,0x00);
writeADV7842(IO_I2C_ADDR,0x03,0x06);
//writeADV7842(IO_I2C_ADDR,0x04,0x82);
//writeADV7842(IO_I2C_ADDR,0x05,0x2c);
writeADV7842(IO_I2C_ADDR,0x0c,0x40);
writeADV7842(IO_I2C_ADDR,0x15,0x80);
writeADV7842(IO_I2C_ADDR,0x19,0x83);
writeADV7842(IO_I2C_ADDR,0x33,0x40);
writeADV7842(AFE_I2C_ADDR,0x00,0x0e);
writeADV7842(AFE_I2C_ADDR,0x02,0x80);
writeADV7842(AFE_I2C_ADDR,0x03,0xb0);
writeADV7842(AFE_I2C_ADDR,0x0c,0x1f);
writeADV7842(AFE_I2C_ADDR,0x12,0x63);
writeADV7842(SDPIO_I2C_ADDR,0x7A,0xa5);
writeADV7842(SDPIO_I2C_ADDR,0x7b,0x8f);
writeADV7842(SDPIO_I2C_ADDR,0x60,0x01);
writeADV7842(SDPIO_I2C_ADDR,0x97,0x00);
writeADV7842(SDPIO_I2C_ADDR,0xb2,0x6c);
writeADV7842(SDPIO_I2C_ADDR,0xe0,0x67);
//writeADV7842(SDPIO_I2C_ADDR,0x67,0x00);
//writeADV7842(SDPIO_I2C_ADDR,0x66,0x22);
//writeADV7842(SDPIO_I2C_ADDR,0xb3,0x20);
writeADV7842(SDP_I2C_ADDR,0x00,0x03);
writeADV7842(SDP_I2C_ADDR,0x01,0x00);
writeADV7842(SDP_I2C_ADDR,0x03,0xe4);
writeADV7842(SDP_I2C_ADDR,0x04,0x0b);
writeADV7842(SDP_I2C_ADDR,0x05,0xc3);
writeADV7842(SDP_I2C_ADDR,0x06,0xfe);
writeADV7842(SDP_I2C_ADDR,0x12,0x05);
writeADV7842(SDP_I2C_ADDR,0x0e,0x3F);
writeADV7842(SDP_I2C_ADDR,0x9a,0x07);
writeADV7842(SDP_I2C_ADDR,0x34,0x80);
writeADV7842(SDP_I2C_ADDR,0xd9,0x00);