Try this way:

from datetime import datetime, time  
# -----------------------------------------------------------------------  
security = symbol('MPC'); start = 1530302400; end = 1530302400 + 86400*14  
# -----------------------------------------------------------------------  
def initialize(context):  
    schedule_function(trade, date_rules.every_day(), time_rules.market_open(minutes = 1) )  
    context.isPurchased = False  
    context.isSelled = False

def trade(context, data):  
    today = get_datetime('America/New_York')  
    # log.info(today)  
    todayTime = datetime.combine(today, time())  
    todayTimeEpoch = int(todayTime.strftime('%s'))  
    # log.info(todayTimeEpoch)  
    if (todayTimeEpoch >=  start) and (todayTimeEpoch <= end):  
        price = data.current(security, 'price')  
        if not context.isPurchased:  
            order_target_percent(security, 1)  
            context.isPurchased = True  
            log.info(price)  
            log.info("buy")  
        if context.isPurchased:  
            if not context.isSelled:  
                if price > 71.91:  
                    order_target_percent(security, 0)  
                    context.isSelled = True  
                    log.info(price)  
                    log.info("sell")  

Thank you Vladimir .
After replacing order with order_target_percent for buying and selling, the return is not 0.
but buy and sell happen many times now.
so, my question is:
1. why order_target_percent works, but order, order_target(I just tried) do not work
2. why the transaction happens many times, I use two bool variables(context.isPurchased, context.isSelled) to make sure the order_target_percent called once(for buy or sell)

buy and sell happen many times now.

1 Try to reduce initial capital to 10000.
2 Use schedule_function instead of handle_data as in my code.

This may help you to understand Quantopian default slippage model.

1 Try to reduce initial capital to 10000.
Yes, now the transactions happen only twice.