at a time 与 at one time 表示“一次；同时”的意思时，可以互换使用，没有区别。例如：

    We cannot do two things at a time / at one time. 我们不能一次[同时]做两件事。

(1)at a time :一次；每次；一次性，同时

Let your listener answer one question at a time.
让你的听众一次回答一个问题

The fish lay thousands of eggs at a time. 

这种鱼一次产卵数千个。

My brain can’t hold so much information at a time. 

我脑子一下子记不住这么多信息。

Webber had three musicals playing in London at a time. 

韦伯有三部音乐剧同时在伦敦上演。

Add the cream a few drops at a time.  

每次加几滴奶油。

For months at a time, air raids were a nightly occurrence.  

一度有几个月，每天晚上都会发生空袭。

Beat in the eggs, one at a time.  

打入鸡蛋，一次一个。

I can only carry two at a time.  

我一次只能带两个。

(2)at one time :曾经，一度；同时，通常用于过去式(表示每次时/一次时，经常可以同at a time 替换）

All showed signs that,at one time,they had been active.
所有迹象表明，它们曾一度活跃过

He was at one time a professor in that university. 

他曾一度是那个大学的教授。

At one time Britainhad colonies all over the world. 

英国曾在全世界都有殖民地。

He was a great lawyer at one time, but he’s over the hill now. 

他曾是一名有名的律师，但是现在不如从前了。

At one time 400 men, women and children lived in the village.  

曾经有400个男人、女人和孩子居住在村子里。

At one time, walking across the room took all her strength.  

曾经有段时间她得费尽全力才能从房间一头走到另一头。

At one time the schools were mainly attached to the church.  

这些学校曾一度归教会主管。

All of them were French possessions at one time or another.  

它们都曾是法国的领地。

She ran for the staircase and down the steps, taking them two at a time 她跑向楼梯，一步两级地奔下台阶。

(1)at one time 一度;曾经（英文解释： If you say that something was the case at one time, you mean that it was the case during a particular period in the past.)

The song had a great vogue at one time.  

这首歌曾经风行一时.

At one time 400 men, women and children lived in the village.  

曾经有 400 个男人、女人和孩子居住在村子里。

(2)at a time 一次;每次 (英文解释：You sayat a time after an amount to say how many things or how much of something is involved in one action, place, or group.)

Beat in the eggs, one at a time. 

打入鸡蛋，一次一个。

She ran for the staircase and down the steps, taking them two at a time.

她跑向楼梯，一步两级地奔下台阶。